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lguest: update commentry

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Every so often, after code shuffles, I need to go through and unbitrot
the Lguest Journey (see drivers/lguest/README).  Since we now use RCU in
a simple form in one place I took the opportunity to expand that explanation.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Rusty Russell 2009-07-30 16:03:45 -06:00
parent 2e04ef7691
commit a91d74a3c4
11 changed files with 399 additions and 112 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

184
Documentation/lguest/lguest.c
View file

@ -49,7 +49,7 @@
#include "linux/virtio_ring.h"
#include "asm/bootparam.h"
/*L:110
* We can ignore the 39 include files we need for this program, but I do want
* We can ignore the 42 include files we need for this program, but I do want
* to draw attention to the use of kernel-style types.
*
* As Linus said, "C is a Spartan language, and so should your naming be." I
@ -305,6 +305,11 @@ static void *map_zeroed_pages(unsigned int num)
PROT_READ|PROT_WRITE|PROT_EXEC, MAP_PRIVATE, fd, 0);
if (addr == MAP_FAILED)
err(1, "Mmaping %u pages of /dev/zero", num);
/*
* One neat mmap feature is that you can close the fd, and it
* stays mapped.
*/
close(fd);
return addr;
@ -557,7 +562,7 @@ static void tell_kernel(unsigned long start)
}
/*:*/
/*
/*L:200
* Device Handling.
*
* When the Guest gives us a buffer, it sends an array of addresses and sizes.
@ -608,7 +613,10 @@ static unsigned next_desc(struct vring_desc *desc,
return next;
}
/* This actually sends the interrupt for this virtqueue */
/*
* This actually sends the interrupt for this virtqueue, if we've used a
* buffer.
*/
static void trigger_irq(struct virtqueue *vq)
{
unsigned long buf[] = { LHREQ_IRQ, vq->config.irq };
@ -629,12 +637,12 @@ static void trigger_irq(struct virtqueue *vq)
}
/*
* This looks in the virtqueue and for the first available buffer, and converts
* This looks in the virtqueue for the first available buffer, and converts
* it to an iovec for convenient access. Since descriptors consist of some
* number of output then some number of input descriptors, it's actually two
* iovecs, but we pack them into one and note how many of each there were.
*
* This function returns the descriptor number found.
* This function waits if necessary, and returns the descriptor number found.
*/
static unsigned wait_for_vq_desc(struct virtqueue *vq,
struct iovec iov[],
@ -644,10 +652,14 @@ static unsigned wait_for_vq_desc(struct virtqueue *vq,
struct vring_desc *desc;
u16 last_avail = lg_last_avail(vq);
/* There's nothing available? */
while (last_avail == vq->vring.avail->idx) {
u64 event;
/* OK, tell Guest about progress up to now. */
/*
* Since we're about to sleep, now is a good time to tell the
* Guest about what we've used up to now.
*/
trigger_irq(vq);
/* OK, now we need to know about added descriptors. */
@ -734,8 +746,9 @@ static unsigned wait_for_vq_desc(struct virtqueue *vq,
}
/*
* After we've used one of their buffers, we tell them about it. We'll then
* want to send them an interrupt, using trigger_irq().
* After we've used one of their buffers, we tell the Guest about it. Sometime
* later we'll want to send them an interrupt using trigger_irq(); note that
* wait_for_vq_desc() does that for us if it has to wait.
*/
static void add_used(struct virtqueue *vq, unsigned int head, int len)
{
@ -782,12 +795,12 @@ static void console_input(struct virtqueue *vq)
struct console_abort *abort = vq->dev->priv;
struct iovec iov[vq->vring.num];
/* Make sure there's a descriptor waiting. */
/* Make sure there's a descriptor available. */
head = wait_for_vq_desc(vq, iov, &out_num, &in_num);
if (out_num)
errx(1, "Output buffers in console in queue?");
/* Read it in. */
/* Read into it. This is where we usually wait. */
len = readv(STDIN_FILENO, iov, in_num);
if (len <= 0) {
/* Ran out of input? */
@ -800,6 +813,7 @@ static void console_input(struct virtqueue *vq)
pause();
}
/* Tell the Guest we used a buffer. */
add_used_and_trigger(vq, head, len);
/*
@ -834,15 +848,23 @@ static void console_output(struct virtqueue *vq)
unsigned int head, out, in;
struct iovec iov[vq->vring.num];
/* We usually wait in here, for the Guest to give us something. */
head = wait_for_vq_desc(vq, iov, &out, &in);
if (in)
errx(1, "Input buffers in console output queue?");
/* writev can return a partial write, so we loop here. */
while (!iov_empty(iov, out)) {
int len = writev(STDOUT_FILENO, iov, out);
if (len <= 0)
err(1, "Write to stdout gave %i", len);
iov_consume(iov, out, len);
}
/*
* We're finished with that buffer: if we're going to sleep,
* wait_for_vq_desc() will prod the Guest with an interrupt.
*/
add_used(vq, head, 0);
}
@ -862,15 +884,30 @@ static void net_output(struct virtqueue *vq)
unsigned int head, out, in;
struct iovec iov[vq->vring.num];
/* We usually wait in here for the Guest to give us a packet. */
head = wait_for_vq_desc(vq, iov, &out, &in);
if (in)
errx(1, "Input buffers in net output queue?");
/*
* Send the whole thing through to /dev/net/tun. It expects the exact
* same format: what a coincidence!
*/
if (writev(net_info->tunfd, iov, out) < 0)
errx(1, "Write to tun failed?");
/*
* Done with that one; wait_for_vq_desc() will send the interrupt if
* all packets are processed.
*/
add_used(vq, head, 0);
}
/* Will reading from this file descriptor block? */
/*
* Handling network input is a bit trickier, because I've tried to optimize it.
*
* First we have a helper routine which tells is if from this file descriptor
* (ie. the /dev/net/tun device) will block:
*/
static bool will_block(int fd)
{
fd_set fdset;
@ -880,7 +917,11 @@ static bool will_block(int fd)
return select(fd+1, &fdset, NULL, NULL, &zero) != 1;
}
/* This handles packets coming in from the tun device to our Guest. */
/*
* This handles packets coming in from the tun device to our Guest. Like all
* service routines, it gets called again as soon as it returns, so you don't
* see a while(1) loop here.
*/
static void net_input(struct virtqueue *vq)
{
int len;
@ -888,21 +929,38 @@ static void net_input(struct virtqueue *vq)
struct iovec iov[vq->vring.num];
struct net_info *net_info = vq->dev->priv;
/*
* Get a descriptor to write an incoming packet into. This will also
* send an interrupt if they're out of descriptors.
*/
head = wait_for_vq_desc(vq, iov, &out, &in);
if (out)
errx(1, "Output buffers in net input queue?");
/* Deliver interrupt now, since we're about to sleep. */
/*
* If it looks like we'll block reading from the tun device, send them
* an interrupt.
*/
if (vq->pending_used && will_block(net_info->tunfd))
trigger_irq(vq);
/*
* Read in the packet. This is where we normally wait (when there's no
* incoming network traffic).
*/
len = readv(net_info->tunfd, iov, in);
if (len <= 0)
err(1, "Failed to read from tun.");
/*
* Mark that packet buffer as used, but don't interrupt here. We want
* to wait until we've done as much work as we can.
*/
add_used(vq, head, len);
}
/*:*/
/* This is the helper to create threads. */
/* This is the helper to create threads: run the service routine in a loop. */
static int do_thread(void *_vq)
{
struct virtqueue *vq = _vq;
@ -950,11 +1008,14 @@ static void reset_device(struct device *dev)
signal(SIGCHLD, (void *)kill_launcher);
}
/*L:216
* This actually creates the thread which services the virtqueue for a device.
*/
static void create_thread(struct virtqueue *vq)
{
/*
* Create stack for thread and run it. Since the stack grows upwards,
* we point the stack pointer to the end of this region.
* Create stack for thread. Since the stack grows upwards, we point
* the stack pointer to the end of this region.
*/
char *stack = malloc(32768);
unsigned long args[] = { LHREQ_EVENTFD,
@ -966,17 +1027,22 @@ static void create_thread(struct virtqueue *vq)
err(1, "Creating eventfd");
args[2] = vq->eventfd;
/* Attach an eventfd to this virtqueue: it will go off
* when the Guest does an LHCALL_NOTIFY for this vq. */
/*
* Attach an eventfd to this virtqueue: it will go off when the Guest
* does an LHCALL_NOTIFY for this vq.
*/
if (write(lguest_fd, &args, sizeof(args)) != 0)
err(1, "Attaching eventfd");
/* CLONE_VM: because it has to access the Guest memory, and
* SIGCHLD so we get a signal if it dies. */
/*
* CLONE_VM: because it has to access the Guest memory, and SIGCHLD so
* we get a signal if it dies.
*/
vq->thread = clone(do_thread, stack + 32768, CLONE_VM | SIGCHLD, vq);
if (vq->thread == (pid_t)-1)
err(1, "Creating clone");
/* We close our local copy, now the child has it. */
/* We close our local copy now the child has it. */
close(vq->eventfd);
}
@ -1028,7 +1094,10 @@ static void update_device_status(struct device *dev)
}
}
/* This is the generic routine we call when the Guest uses LHCALL_NOTIFY. */
/*L:215
* This is the generic routine we call when the Guest uses LHCALL_NOTIFY. In
* particular, it's used to notify us of device status changes during boot.
*/
static void handle_output(unsigned long addr)
{
struct device *i;
@ -1037,18 +1106,32 @@ static void handle_output(unsigned long addr)
for (i = devices.dev; i; i = i->next) {
struct virtqueue *vq;
/* Notifications to device descriptors update device status. */
/*
* Notifications to device descriptors mean they updated the
* device status.
*/
if (from_guest_phys(addr) == i->desc) {
update_device_status(i);
return;
}
/* Devices *can* be used before status is set to DRIVER_OK. */
/*
* Devices *can* be used before status is set to DRIVER_OK.
* The original plan was that they would never do this: they
* would always finish setting up their status bits before
* actually touching the virtqueues. In practice, we allowed
* them to, and they do (eg. the disk probes for partition
* tables as part of initialization).
*
* If we see this, we start the device: once it's running, we
* expect the device to catch all the notifications.
*/
for (vq = i->vq; vq; vq = vq->next) {
if (addr != vq->config.pfn*getpagesize())
continue;
if (i->running)
errx(1, "Notification on running %s", i->name);
/* This just calls create_thread() for each virtqueue */
start_device(i);
return;
}
@ -1132,6 +1215,11 @@ static void add_virtqueue(struct device *dev, unsigned int num_descs,
vq->next = NULL;
vq->last_avail_idx = 0;
vq->dev = dev;
/*
* This is the routine the service thread will run, and its Process ID
* once it's running.
*/
vq->service = service;
vq->thread = (pid_t)-1;
@ -1202,7 +1290,8 @@ static void set_config(struct device *dev, unsigned len, const void *conf)
/*
* This routine does all the creation and setup of a new device, including
* calling new_dev_desc() to allocate the descriptor and device memory.
* calling new_dev_desc() to allocate the descriptor and device memory. We
* don't actually start the service threads until later.
*
* See what I mean about userspace being boring?
*/
@ -1478,19 +1567,7 @@ static void setup_tun_net(char *arg)
verbose("device %u: tun %s: %s\n",
devices.device_num, tapif, arg);
}
/*
* Our block (disk) device should be really simple: the Guest asks for a block
* number and we read or write that position in the file. Unfortunately, that
* was amazingly slow: the Guest waits until the read is finished before
* running anything else, even if it could have been doing useful work.
*
* We could use async I/O, except it's reputed to suck so hard that characters
* actually go missing from your code when you try to use it.
*
* So this was one reason why lguest now does all virtqueue servicing in
* separate threads: it's more efficient and more like a real device.
*/
/*:*/
/* This hangs off device->priv. */
struct vblk_info
@ -1512,8 +1589,16 @@ struct vblk_info
/*L:210
* The Disk
*
* Remember that the block device is handled by a separate I/O thread. We head
* straight into the core of that thread here:
* The disk only has one virtqueue, so it only has one thread. It is really
* simple: the Guest asks for a block number and we read or write that position
* in the file.
*
* Before we serviced each virtqueue in a separate thread, that was unacceptably
* slow: the Guest waits until the read is finished before running anything
* else, even if it could have been doing useful work.
*
* We could have used async I/O, except it's reputed to suck so hard that
* characters actually go missing from your code when you try to use it.
*/
static void blk_request(struct virtqueue *vq)
{
@ -1525,7 +1610,10 @@ static void blk_request(struct virtqueue *vq)
struct iovec iov[vq->vring.num];
off64_t off;
/* Get the next request. */
/*
* Get the next request, where we normally wait. It triggers the
* interrupt to acknowledge previously serviced requests (if any).
*/
head = wait_for_vq_desc(vq, iov, &out_num, &in_num);
/*
@ -1539,6 +1627,10 @@ static void blk_request(struct virtqueue *vq)
out = convert(&iov[0], struct virtio_blk_outhdr);
in = convert(&iov[out_num+in_num-1], u8);
/*
* For historical reasons, block operations are expressed in 512 byte
* "sectors".
*/
off = out->sector * 512;
/*
@ -1614,6 +1706,7 @@ static void blk_request(struct virtqueue *vq)
if (out->type & VIRTIO_BLK_T_BARRIER)
fdatasync(vblk->fd);
/* Finished that request. */
add_used(vq, head, wlen);
}
@ -1682,9 +1775,8 @@ static void rng_input(struct virtqueue *vq)
errx(1, "Output buffers in rng?");
/*
* This is why we convert to iovecs: the readv() call uses them, and so
* it reads straight into the Guest's buffer. We loop to make sure we
* fill it.
* Just like the console write, we loop to cover the whole iovec.
* In this case, short reads actually happen quite a bit.
*/
while (!iov_empty(iov, in_num)) {
len = readv(rng_info->rfd, iov, in_num);
@ -1818,7 +1910,9 @@ int main(int argc, char *argv[])
devices.lastdev = NULL;
devices.next_irq = 1;
/* We're CPU 0. In fact, that's the only CPU possible right now. */
cpu_id = 0;
/*
* We need to know how much memory so we can set up the device
* descriptor and memory pages for the devices as we parse the command
@ -1926,7 +2020,7 @@ int main(int argc, char *argv[])
*/
tell_kernel(start);
/* Ensure that we terminate if a child dies. */
/* Ensure that we terminate if a device-servicing child dies. */
signal(SIGCHLD, kill_launcher);
/* If we exit via err(), this kills all the threads, restores tty. */

8
arch/x86/include/asm/lguest_hcall.h
View file

@ -35,10 +35,10 @@
* operations? There are two ways: the direct way is to make a "hypercall",
* to make requests of the Host Itself.
*
* We use the KVM hypercall mechanism. Seventeen hypercalls are
* available: the hypercall number is put in the %eax register, and the
* arguments (when required) are placed in %ebx, %ecx, %edx and %esi.
* If a return value makes sense, it's returned in %eax.
* We use the KVM hypercall mechanism, though completely different hypercall
* numbers. Seventeen hypercalls are available: the hypercall number is put in
* the %eax register, and the arguments (when required) are placed in %ebx,
* %ecx, %edx and %esi. If a return value makes sense, it's returned in %eax.
*
* Grossly invalid calls result in Sudden Death at the hands of the vengeful
* Host, rather than returning failure. This reflects Winston Churchill's

99
arch/x86/lguest/boot.c
View file

@ -154,6 +154,7 @@ static void lazy_hcall1(unsigned long call,
async_hcall(call, arg1, 0, 0, 0);
}
/* You can imagine what lazy_hcall2, 3 and 4 look like. :*/
static void lazy_hcall2(unsigned long call,
unsigned long arg1,
unsigned long arg2)
@ -189,8 +190,10 @@ static void lazy_hcall4(unsigned long call,
}
#endif
/* When lazy mode is turned off reset the per-cpu lazy mode variable and then
* issue the do-nothing hypercall to flush any stored calls. */
/*G:036
* When lazy mode is turned off reset the per-cpu lazy mode variable and then
* issue the do-nothing hypercall to flush any stored calls.
:*/
static void lguest_leave_lazy_mmu_mode(void)
{
kvm_hypercall0(LHCALL_FLUSH_ASYNC);
@ -250,13 +253,11 @@ extern void lg_irq_enable(void);
extern void lg_restore_fl(unsigned long flags);
/*M:003
* Note that we don't check for outstanding interrupts when we re-enable them
* (or when we unmask an interrupt). This seems to work for the moment, since
* interrupts are rare and we'll just get the interrupt on the next timer tick,
* but now we can run with CONFIG_NO_HZ, we should revisit this. One way would
* be to put the "irq_enabled" field in a page by itself, and have the Host
* write-protect it when an interrupt comes in when irqs are disabled. There
* will then be a page fault as soon as interrupts are re-enabled.
* We could be more efficient in our checking of outstanding interrupts, rather
* than using a branch. One way would be to put the "irq_enabled" field in a
* page by itself, and have the Host write-protect it when an interrupt comes
* in when irqs are disabled. There will then be a page fault as soon as
* interrupts are re-enabled.
*
* A better method is to implement soft interrupt disable generally for x86:
* instead of disabling interrupts, we set a flag. If an interrupt does come
@ -568,7 +569,7 @@ static void lguest_write_cr4(unsigned long val)
* cr3 ---> +---------+
* | --------->+---------+
* | | | PADDR1 |
* Top-level | | PADDR2 |
* Mid-level | | PADDR2 |
* (PMD) page | | |
* | | Lower-level |
* | | (PTE) page |
@ -588,23 +589,62 @@ static void lguest_write_cr4(unsigned long val)
* Index into top Index into second Offset within page
* page directory page pagetable page
*
* The kernel spends a lot of time changing both the top-level page directory
* and lower-level pagetable pages. The Guest doesn't know physical addresses,
* so while it maintains these page tables exactly like normal, it also needs
* to keep the Host informed whenever it makes a change: the Host will create
* the real page tables based on the Guests'.
* Now, unfortunately, this isn't the whole story: Intel added Physical Address
* Extension (PAE) to allow 32 bit systems to use 64GB of memory (ie. 36 bits).
* These are held in 64-bit page table entries, so we can now only fit 512
* entries in a page, and the neat three-level tree breaks down.
*
* The result is a four level page table:
*
* cr3 --> [ 4 Upper ]
* [ Level ]
* [ Entries ]
* [(PUD Page)]---> +---------+
* | --------->+---------+
* | | | PADDR1 |
* Mid-level | | PADDR2 |
* (PMD) page | | |
* | | Lower-level |
* | | (PTE) page |
* | | | |
* .... ....
*
*
* And the virtual address is decoded as:
*
* 1 1 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
* |<-2->|<--- 9 bits ---->|<---- 9 bits --->|<------ 12 bits ------>|
* Index into Index into mid Index into lower Offset within page
* top entries directory page pagetable page
*
* It's too hard to switch between these two formats at runtime, so Linux only
* supports one or the other depending on whether CONFIG_X86_PAE is set. Many
* distributions turn it on, and not just for people with silly amounts of
* memory: the larger PTE entries allow room for the NX bit, which lets the
* kernel disable execution of pages and increase security.
*
* This was a problem for lguest, which couldn't run on these distributions;
* then Matias Zabaljauregui figured it all out and implemented it, and only a
* handful of puppies were crushed in the process!
*
* Back to our point: the kernel spends a lot of time changing both the
* top-level page directory and lower-level pagetable pages. The Guest doesn't
* know physical addresses, so while it maintains these page tables exactly
* like normal, it also needs to keep the Host informed whenever it makes a
* change: the Host will create the real page tables based on the Guests'.
*/
/*
* The Guest calls this to set a second-level entry (pte), ie. to map a page
* into a process' address space. We set the entry then tell the Host the
* toplevel and address this corresponds to. The Guest uses one pagetable per
* process, so we need to tell the Host which one we're changing (mm->pgd).
* The Guest calls this after it has set a second-level entry (pte), ie. to map
* a page into a process' address space. Wetell the Host the toplevel and
* address this corresponds to. The Guest uses one pagetable per process, so
* we need to tell the Host which one we're changing (mm->pgd).
*/
static void lguest_pte_update(struct mm_struct *mm, unsigned long addr,
pte_t *ptep)
{
#ifdef CONFIG_X86_PAE
/* PAE needs to hand a 64 bit page table entry, so it uses two args. */
lazy_hcall4(LHCALL_SET_PTE, __pa(mm->pgd), addr,
ptep->pte_low, ptep->pte_high);
#else
@ -612,6 +652,7 @@ static void lguest_pte_update(struct mm_struct *mm, unsigned long addr,
#endif
}
/* This is the "set and update" combo-meal-deal version. */
static void lguest_set_pte_at(struct mm_struct *mm, unsigned long addr,
pte_t *ptep, pte_t pteval)
{
@ -672,6 +713,11 @@ static void lguest_set_pte(pte_t *ptep, pte_t pteval)
}
#ifdef CONFIG_X86_PAE
/*
* With 64-bit PTE values, we need to be careful setting them: if we set 32
* bits at a time, the hardware could see a weird half-set entry. These
* versions ensure we update all 64 bits at once.
*/
static void lguest_set_pte_atomic(pte_t *ptep, pte_t pte)
{
native_set_pte_atomic(ptep, pte);
@ -679,13 +725,14 @@ static void lguest_set_pte_atomic(pte_t *ptep, pte_t pte)
lazy_hcall1(LHCALL_FLUSH_TLB, 1);
}
void lguest_pte_clear(struct mm_struct *mm, unsigned long addr, pte_t *ptep)
static void lguest_pte_clear(struct mm_struct *mm, unsigned long addr,
pte_t *ptep)
{
native_pte_clear(mm, addr, ptep);
lguest_pte_update(mm, addr, ptep);
}
void lguest_pmd_clear(pmd_t *pmdp)
static void lguest_pmd_clear(pmd_t *pmdp)
{
lguest_set_pmd(pmdp, __pmd(0));
}
@ -784,6 +831,14 @@ static void __init lguest_init_IRQ(void)
irq_ctx_init(smp_processor_id());
}
/*
* With CONFIG_SPARSE_IRQ, interrupt descriptors are allocated as-needed, so
* rather than set them in lguest_init_IRQ we are called here every time an
* lguest device needs an interrupt.
*
* FIXME: irq_to_desc_alloc_node() can fail due to lack of memory, we should
* pass that up!
*/
void lguest_setup_irq(unsigned int irq)
{
irq_to_desc_alloc_node(irq, 0);
@ -1298,7 +1353,7 @@ __init void lguest_init(void)
*/
switch_to_new_gdt(0);
/* As described in head_32.S, we map the first 128M of memory. */
/* We actually boot with all memory mapped, but let's say 128MB. */
max_pfn_mapped = (128*1024*1024) >> PAGE_SHIFT;
/*

2
arch/x86/lguest/i386_head.S
View file

@ -102,6 +102,7 @@ send_interrupts:
* create one manually here.
*/
.byte 0x0f,0x01,0xc1 /* KVM_HYPERCALL */
/* Put eax back the way we found it. */
popl %eax
ret
@ -125,6 +126,7 @@ ENTRY(lg_restore_fl)
jnz send_interrupts
/* Again, the normal path has used no extra registers. Clever, huh? */
ret
/*:*/
/* These demark the EIP range where host should never deliver interrupts. */
.global lguest_noirq_start

7
drivers/lguest/core.c
View file

@ -217,10 +217,15 @@ int run_guest(struct lg_cpu *cpu, unsigned long __user *user)
/*
* It's possible the Guest did a NOTIFY hypercall to the
* Launcher, in which case we return from the read() now.
* Launcher.
*/
if (cpu->pending_notify) {
/*
* Does it just needs to write to a registered
* eventfd (ie. the appropriate virtqueue thread)?
*/
if (!send_notify_to_eventfd(cpu)) {
/* OK, we tell the main Laucher. */
if (put_user(cpu->pending_notify, user))
return -EFAULT;
return sizeof(cpu->pending_notify);

6
drivers/lguest/hypercalls.c
View file

@ -59,7 +59,7 @@ static void do_hcall(struct lg_cpu *cpu, struct hcall_args *args)
case LHCALL_SHUTDOWN: {
char msg[128];
/*
* Shutdown is such a trivial hypercall that we do it in four
* Shutdown is such a trivial hypercall that we do it in five
* lines right here.
*
* If the lgread fails, it will call kill_guest() itself; the
@ -245,6 +245,10 @@ static void initialize(struct lg_cpu *cpu)
* device), the Guest will still see the old page. In practice, this never
* happens: why would the Guest read a page which it has never written to? But
* a similar scenario might one day bite us, so it's worth mentioning.
*
* Note that if we used a shared anonymous mapping in the Launcher instead of
* mapping /dev/zero private, we wouldn't worry about cop-on-write. And we
* need that to switch the Launcher to processes (away from threads) anyway.
:*/
/*H:100

11
drivers/lguest/lguest_device.c
View file

@ -236,7 +236,7 @@ static void lg_notify(struct virtqueue *vq)
extern void lguest_setup_irq(unsigned int irq);
/*
* This routine finds the first virtqueue described in the configuration of
* This routine finds the Nth virtqueue described in the configuration of
* this device and sets it up.
*
* This is kind of an ugly duckling. It'd be nicer to have a standard
@ -244,9 +244,6 @@ extern void lguest_setup_irq(unsigned int irq);
* everyone wants to do it differently. The KVM coders want the Guest to
* allocate its own pages and tell the Host where they are, but for lguest it's
* simpler for the Host to simply tell us where the pages are.
*
* So we provide drivers with a "find the Nth virtqueue and set it up"
* function.
*/
static struct virtqueue *lg_find_vq(struct virtio_device *vdev,
unsigned index,
@ -422,7 +419,11 @@ static void add_lguest_device(struct lguest_device_desc *d,
/* This devices' parent is the lguest/ dir. */
ldev->vdev.dev.parent = lguest_root;
/* We have a unique device index thanks to the dev_index counter. */
/*
* The device type comes straight from the descriptor. There's also a
* device vendor field in the virtio_device struct, which we leave as
* 0.
*/
ldev->vdev.id.device = d->type;
/*
* We have a simple set of routines for querying the device's

100
drivers/lguest/lguest_user.c
View file

@ -1,9 +1,8 @@
/*P:200
* This contains all the /dev/lguest code, whereby the userspace launcher
/*P:200 This contains all the /dev/lguest code, whereby the userspace launcher
* controls and communicates with the Guest. For example, the first write will
* tell us the Guest's memory layout, pagetable, entry point and kernel address
* offset. A read will run the Guest until something happens, such as a signal
* or the Guest doing a NOTIFY out to the Launcher.
* tell us the Guest's memory layout and entry point. A read will run the
* Guest until something happens, such as a signal or the Guest doing a NOTIFY
* out to the Launcher.
:*/
#include <linux/uaccess.h>
#include <linux/miscdevice.h>
@ -13,14 +12,41 @@
#include <linux/file.h>
#include "lg.h"
/*L:056
* Before we move on, let's jump ahead and look at what the kernel does when
* it needs to look up the eventfds. That will complete our picture of how we
* use RCU.
*
* The notification value is in cpu->pending_notify: we return true if it went
* to an eventfd.
*/
bool send_notify_to_eventfd(struct lg_cpu *cpu)
{
unsigned int i;
struct lg_eventfd_map *map;
/* lg->eventfds is RCU-protected */
/*
* This "rcu_read_lock()" helps track when someone is still looking at
* the (RCU-using) eventfds array. It's not actually a lock at all;
* indeed it's a noop in many configurations. (You didn't expect me to
* explain all the RCU secrets here, did you?)
*/
rcu_read_lock();
/*
* rcu_dereference is the counter-side of rcu_assign_pointer(); it
* makes sure we don't access the memory pointed to by
* cpu->lg->eventfds before cpu->lg->eventfds is set. Sounds crazy,
* but Alpha allows this! Paul McKenney points out that a really
* aggressive compiler could have the same effect:
* http://lists.ozlabs.org/pipermail/lguest/2009-July/001560.html
*
* So play safe, use rcu_dereference to get the rcu-protected pointer:
*/
map = rcu_dereference(cpu->lg->eventfds);
/*
* Simple array search: even if they add an eventfd while we do this,
* we'll continue to use the old array and just won't see the new one.
*/
for (i = 0; i < map->num; i++) {
if (map->map[i].addr == cpu->pending_notify) {
eventfd_signal(map->map[i].event, 1);
@ -28,14 +54,43 @@ bool send_notify_to_eventfd(struct lg_cpu *cpu)
break;
}
}
/* We're done with the rcu-protected variable cpu->lg->eventfds. */
rcu_read_unlock();
/* If we cleared the notification, it's because we found a match. */
return cpu->pending_notify == 0;
}
/*L:055
* One of the more tricksy tricks in the Linux Kernel is a technique called
* Read Copy Update. Since one point of lguest is to teach lguest journeyers
* about kernel coding, I use it here. (In case you're curious, other purposes
* include learning about virtualization and instilling a deep appreciation for
* simplicity and puppies).
*
* We keep a simple array which maps LHCALL_NOTIFY values to eventfds, but we
* add new eventfds without ever blocking readers from accessing the array.
* The current Launcher only does this during boot, so that never happens. But
* Read Copy Update is cool, and adding a lock risks damaging even more puppies
* than this code does.
*
* We allocate a brand new one-larger array, copy the old one and add our new
* element. Then we make the lg eventfd pointer point to the new array.
* That's the easy part: now we need to free the old one, but we need to make
* sure no slow CPU somewhere is still looking at it. That's what
* synchronize_rcu does for us: waits until every CPU has indicated that it has
* moved on to know it's no longer using the old one.
*
* If that's unclear, see http://en.wikipedia.org/wiki/Read-copy-update.
*/
static int add_eventfd(struct lguest *lg, unsigned long addr, int fd)
{
struct lg_eventfd_map *new, *old = lg->eventfds;
/*
* We don't allow notifications on value 0 anyway (pending_notify of
* 0 means "nothing pending").
*/
if (!addr)
return -EINVAL;
@ -62,12 +117,20 @@ static int add_eventfd(struct lguest *lg, unsigned long addr, int fd)
}
new->num++;
/* Now put new one in place. */
/*
* Now put new one in place: rcu_assign_pointer() is a fancy way of
* doing "lg->eventfds = new", but it uses memory barriers to make
* absolutely sure that the contents of "new" written above is nailed
* down before we actually do the assignment.
*
* We have to think about these kinds of things when we're operating on
* live data without locks.
*/
rcu_assign_pointer(lg->eventfds, new);
/*
* We're not in a big hurry. Wait until noone's looking at old
* version, then delete it.
* version, then free it.
*/
synchronize_rcu();
kfree(old);
@ -75,6 +138,14 @@ static int add_eventfd(struct lguest *lg, unsigned long addr, int fd)
return 0;
}
/*L:052
* Receiving notifications from the Guest is usually done by attaching a
* particular LHCALL_NOTIFY value to an event filedescriptor. The eventfd will
* become readable when the Guest does an LHCALL_NOTIFY with that value.
*
* This is really convenient for processing each virtqueue in a separate
* thread.
*/
static int attach_eventfd(struct lguest *lg, const unsigned long __user *input)
{
unsigned long addr, fd;
@ -86,6 +157,11 @@ static int attach_eventfd(struct lguest *lg, const unsigned long __user *input)
if (get_user(fd, input) != 0)
return -EFAULT;
/*
* Just make sure two callers don't add eventfds at once. We really
* only need to lock against callers adding to the same Guest, so using
* the Big Lguest Lock is overkill. But this is setup, not a fast path.
*/
mutex_lock(&lguest_lock);
err = add_eventfd(lg, addr, fd);
mutex_unlock(&lguest_lock);
@ -106,6 +182,10 @@ static int user_send_irq(struct lg_cpu *cpu, const unsigned long __user *input)
if (irq >= LGUEST_IRQS)
return -EINVAL;
/*
* Next time the Guest runs, the core code will see if it can deliver
* this interrupt.
*/
set_interrupt(cpu, irq);
return 0;
}
@ -307,10 +387,10 @@ unlock:
* The first operation the Launcher does must be a write. All writes
* start with an unsigned long number: for the first write this must be
* LHREQ_INITIALIZE to set up the Guest. After that the Launcher can use
* writes of other values to send interrupts.
* writes of other values to send interrupts or set up receipt of notifications.
*
* Note that we overload the "offset" in the /dev/lguest file to indicate what
* CPU number we're dealing with. Currently this is always 0, since we only
* CPU number we're dealing with. Currently this is always 0 since we only
* support uniprocessor Guests, but you can see the beginnings of SMP support
* here.
*/

86
drivers/lguest/page_tables.c
View file

@ -29,10 +29,10 @@
/*H:300
* The Page Table Code
*
* We use two-level page tables for the Guest. If you're not entirely
* comfortable with virtual addresses, physical addresses and page tables then
* I recommend you review arch/x86/lguest/boot.c's "Page Table Handling" (with
* diagrams!).
* We use two-level page tables for the Guest, or three-level with PAE. If
* you're not entirely comfortable with virtual addresses, physical addresses
* and page tables then I recommend you review arch/x86/lguest/boot.c's "Page
* Table Handling" (with diagrams!).
*
* The Guest keeps page tables, but we maintain the actual ones here: these are
* called "shadow" page tables. Which is a very Guest-centric name: these are
@ -52,9 +52,8 @@
:*/
/*
* 1024 entries in a page table page maps 1024 pages: 4MB. The Switcher is
* conveniently placed at the top 4MB, so it uses a separate, complete PTE
* page.
* The Switcher uses the complete top PTE page. That's 1024 PTE entries (4MB)
* or 512 PTE entries with PAE (2MB).
*/
#define SWITCHER_PGD_INDEX (PTRS_PER_PGD - 1)
@ -81,7 +80,8 @@ static DEFINE_PER_CPU(pte_t *, switcher_pte_pages);
/*H:320
* The page table code is curly enough to need helper functions to keep it
* clear and clean.
* clear and clean. The kernel itself provides many of them; one advantage
* of insisting that the Guest and Host use the same CONFIG_PAE setting.
*
* There are two functions which return pointers to the shadow (aka "real")
* page tables.
@ -155,7 +155,7 @@ static pte_t *spte_addr(struct lg_cpu *cpu, pgd_t spgd, unsigned long vaddr)
}
/*
* These two functions just like the above two, except they access the Guest
* These functions are just like the above two, except they access the Guest
* page tables. Hence they return a Guest address.
*/
static unsigned long gpgd_addr(struct lg_cpu *cpu, unsigned long vaddr)
@ -165,6 +165,7 @@ static unsigned long gpgd_addr(struct lg_cpu *cpu, unsigned long vaddr)
}
#ifdef CONFIG_X86_PAE
/* Follow the PGD to the PMD. */
static unsigned long gpmd_addr(pgd_t gpgd, unsigned long vaddr)
{
unsigned long gpage = pgd_pfn(gpgd) << PAGE_SHIFT;
@ -172,6 +173,7 @@ static unsigned long gpmd_addr(pgd_t gpgd, unsigned long vaddr)
return gpage + pmd_index(vaddr) * sizeof(pmd_t);
}
/* Follow the PMD to the PTE. */
static unsigned long gpte_addr(struct lg_cpu *cpu,
pmd_t gpmd, unsigned long vaddr)
{
@ -181,6 +183,7 @@ static unsigned long gpte_addr(struct lg_cpu *cpu,
return gpage + pte_index(vaddr) * sizeof(pte_t);
}
#else
/* Follow the PGD to the PTE (no mid-level for !PAE). */
static unsigned long gpte_addr(struct lg_cpu *cpu,
pgd_t gpgd, unsigned long vaddr)
{
@ -314,6 +317,7 @@ bool demand_page(struct lg_cpu *cpu, unsigned long vaddr, int errcode)
pte_t gpte;
pte_t *spte;
/* Mid level for PAE. */
#ifdef CONFIG_X86_PAE
pmd_t *spmd;
pmd_t gpmd;
@ -391,6 +395,8 @@ bool demand_page(struct lg_cpu *cpu, unsigned long vaddr, int errcode)
*/
gpte_ptr = gpte_addr(cpu, gpgd, vaddr);
#endif
/* Read the actual PTE value. */
gpte = lgread(cpu, gpte_ptr, pte_t);
/* If this page isn't in the Guest page tables, we can't page it in. */
@ -507,6 +513,7 @@ void pin_page(struct lg_cpu *cpu, unsigned long vaddr)
if (!page_writable(cpu, vaddr) && !demand_page(cpu, vaddr, 2))
kill_guest(cpu, "bad stack page %#lx", vaddr);
}
/*:*/
#ifdef CONFIG_X86_PAE
static void release_pmd(pmd_t *spmd)
@ -543,7 +550,11 @@ static void release_pgd(pgd_t *spgd)
}
#else /* !CONFIG_X86_PAE */
/*H:450 If we chase down the release_pgd() code, it looks like this: */
/*H:450
* If we chase down the release_pgd() code, the non-PAE version looks like
* this. The PAE version is almost identical, but instead of calling
* release_pte it calls release_pmd(), which looks much like this.
*/
static void release_pgd(pgd_t *spgd)
{
/* If the entry's not present, there's nothing to release. */
@ -898,17 +909,21 @@ void guest_set_pgd(struct lguest *lg, unsigned long gpgdir, u32 idx)
/* ... throw it away. */
release_pgd(lg->pgdirs[pgdir].pgdir + idx);
}
#ifdef CONFIG_X86_PAE
/* For setting a mid-level, we just throw everything away. It's easy. */
void guest_set_pmd(struct lguest *lg, unsigned long pmdp, u32 idx)
{
guest_pagetable_clear_all(&lg->cpus[0]);
}
#endif
/*
* Once we know how much memory we have we can construct simple identity (which
/*H:505
* To get through boot, we construct simple identity page mappings (which
* set virtual == physical) and linear mappings which will get the Guest far
* enough into the boot to create its own.
* enough into the boot to create its own. The linear mapping means we
* simplify the Guest boot, but it makes assumptions about their PAGE_OFFSET,
* as you'll see.
*
* We lay them out of the way, just below the initrd (which is why we need to
* know its size here).
@ -944,6 +959,10 @@ static unsigned long setup_pagetables(struct lguest *lg,
linear = (void *)pgdir - linear_pages * PAGE_SIZE;
#ifdef CONFIG_X86_PAE
/*
* And the single mid page goes below that. We only use one, but
* that's enough to map 1G, which definitely gets us through boot.
*/
pmds = (void *)linear - PAGE_SIZE;
#endif
/*
@ -957,13 +976,14 @@ static unsigned long setup_pagetables(struct lguest *lg,
return -EFAULT;
}
/*
* The top level points to the linear page table pages above.
* We setup the identity and linear mappings here.
*/
#ifdef CONFIG_X86_PAE
/*
* Make the Guest PMD entries point to the corresponding place in the
* linear mapping (up to one page worth of PMD).
*/
for (i = j = 0; i < mapped_pages && j < PTRS_PER_PMD;
i += PTRS_PER_PTE, j++) {
/* FIXME: native_set_pmd is overkill here. */
native_set_pmd(&pmd, __pmd(((unsigned long)(linear + i)
- mem_base) | _PAGE_PRESENT | _PAGE_RW | _PAGE_USER));
@ -971,18 +991,36 @@ static unsigned long setup_pagetables(struct lguest *lg,
return -EFAULT;
}
/* One PGD entry, pointing to that PMD page. */
set_pgd(&pgd, __pgd(((u32)pmds - mem_base) | _PAGE_PRESENT));
/* Copy it in as the first PGD entry (ie. addresses 0-1G). */
if (copy_to_user(&pgdir[0], &pgd, sizeof(pgd)) != 0)
return -EFAULT;
/*
* And the third PGD entry (ie. addresses 3G-4G).
*
* FIXME: This assumes that PAGE_OFFSET for the Guest is 0xC0000000.
*/
if (copy_to_user(&pgdir[3], &pgd, sizeof(pgd)) != 0)
return -EFAULT;
#else
/*
* The top level points to the linear page table pages above.
* We setup the identity and linear mappings here.
*/
phys_linear = (unsigned long)linear - mem_base;
for (i = 0; i < mapped_pages; i += PTRS_PER_PTE) {
pgd_t pgd;
/*
* Create a PGD entry which points to the right part of the
* linear PTE pages.
*/
pgd = __pgd((phys_linear + i * sizeof(pte_t)) |
(_PAGE_PRESENT | _PAGE_RW | _PAGE_USER));
/*
* Copy it into the PGD page at 0 and PAGE_OFFSET.
*/
if (copy_to_user(&pgdir[i / PTRS_PER_PTE], &pgd, sizeof(pgd))
|| copy_to_user(&pgdir[pgd_index(PAGE_OFFSET)
+ i / PTRS_PER_PTE],
@ -992,8 +1030,8 @@ static unsigned long setup_pagetables(struct lguest *lg,
#endif
/*
* We return the top level (guest-physical) address: remember where
* this is.
* We return the top level (guest-physical) address: we remember where
* this is to write it into lguest_data when the Guest initializes.
*/
return (unsigned long)pgdir - mem_base;
}
@ -1031,7 +1069,9 @@ int init_guest_pagetable(struct lguest *lg)
lg->pgdirs[0].pgdir = (pgd_t *)get_zeroed_page(GFP_KERNEL);
if (!lg->pgdirs[0].pgdir)
return -ENOMEM;
#ifdef CONFIG_X86_PAE
/* For PAE, we also create the initial mid-level. */
pgd = lg->pgdirs[0].pgdir;
pmd_table = (pmd_t *) get_zeroed_page(GFP_KERNEL);
if (!pmd_table)
@ -1040,11 +1080,13 @@ int init_guest_pagetable(struct lguest *lg)
set_pgd(pgd + SWITCHER_PGD_INDEX,
__pgd(__pa(pmd_table) | _PAGE_PRESENT));
#endif
/* This is the current page table. */
lg->cpus[0].cpu_pgd = 0;
return 0;
}
/* When the Guest calls LHCALL_LGUEST_INIT we do more setup. */
/*H:508 When the Guest calls LHCALL_LGUEST_INIT we do more setup. */
void page_table_guest_data_init(struct lg_cpu *cpu)
{
/* We get the kernel address: above this is all kernel memory. */
@ -1105,12 +1147,16 @@ void map_switcher_in_guest(struct lg_cpu *cpu, struct lguest_pages *pages)
pmd_t switcher_pmd;
pmd_t *pmd_table;
/* FIXME: native_set_pmd is overkill here. */
native_set_pmd(&switcher_pmd, pfn_pmd(__pa(switcher_pte_page) >>
PAGE_SHIFT, PAGE_KERNEL_EXEC));
/* Figure out where the pmd page is, by reading the PGD, and converting
* it to a virtual address. */
pmd_table = __va(pgd_pfn(cpu->lg->
pgdirs[cpu->cpu_pgd].pgdir[SWITCHER_PGD_INDEX])
<< PAGE_SHIFT);
/* Now write it into the shadow page table. */
native_set_pmd(&pmd_table[SWITCHER_PMD_INDEX], switcher_pmd);
#else
pgd_t switcher_pgd;

2
drivers/lguest/x86/core.c
View file

@ -187,7 +187,7 @@ static void run_guest_once(struct lg_cpu *cpu, struct lguest_pages *pages)
* also simplify copy_in_guest_info(). Note that we'd still need to restore
* things when we exit to Launcher userspace, but that's fairly easy.
*
* We could also try using this hooks for PGE, but that might be too expensive.
* We could also try using these hooks for PGE, but that might be too expensive.
*
* The hooks were designed for KVM, but we can also put them to good use.
:*/

6
drivers/lguest/x86/switcher_32.S
View file

@ -1,7 +1,7 @@
/*P:900
* This is the Switcher: code which sits at 0xFFC00000 astride both the
* Host and Guest to do the low-level Guest<->Host switch. It is as simple as
* it can be made, but it's naturally very specific to x86.
* This is the Switcher: code which sits at 0xFFC00000 (or 0xFFE00000) astride
* both the Host and Guest to do the low-level Guest<->Host switch. It is as
* simple as it can be made, but it's naturally very specific to x86.
*
* You have now completed Preparation. If this has whet your appetite; if you
* are feeling invigorated and refreshed then the next, more challenging stage