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[IA64] pvops: documentation on ia64/pv_ops
Documentation on ia64/pv_ops which describes its strategy and implementation. Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp> Cc: Gerald Pfeifer <gp@novell.com> Signed-off-by: Tony Luck <tony.luck@intel.com>
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Documentation/ia64/paravirt_ops.txt
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Documentation/ia64/paravirt_ops.txt
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Paravirt_ops on IA64
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====================
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21 May 2008, Isaku Yamahata <yamahata@valinux.co.jp>
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Introduction
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------------
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The aim of this documentation is to help with maintainability and/or to
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encourage people to use paravirt_ops/IA64.
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paravirt_ops (pv_ops in short) is a way for virtualization support of
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Linux kernel on x86. Several ways for virtualization support were
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proposed, paravirt_ops is the winner.
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On the other hand, now there are also several IA64 virtualization
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technologies like kvm/IA64, xen/IA64 and many other academic IA64
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hypervisors so that it is good to add generic virtualization
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infrastructure on Linux/IA64.
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What is paravirt_ops?
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---------------------
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It has been developed on x86 as virtualization support via API, not ABI.
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It allows each hypervisor to override operations which are important for
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hypervisors at API level. And it allows a single kernel binary to run on
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all supported execution environments including native machine.
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Essentially paravirt_ops is a set of function pointers which represent
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operations corresponding to low level sensitive instructions and high
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level functionalities in various area. But one significant difference
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from usual function pointer table is that it allows optimization with
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binary patch. It is because some of these operations are very
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performance sensitive and indirect call overhead is not negligible.
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With binary patch, indirect C function call can be transformed into
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direct C function call or in-place execution to eliminate the overhead.
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Thus, operations of paravirt_ops are classified into three categories.
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- simple indirect call
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These operations correspond to high level functionality so that the
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overhead of indirect call isn't very important.
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- indirect call which allows optimization with binary patch
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Usually these operations correspond to low level instructions. They
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are called frequently and performance critical. So the overhead is
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very important.
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- a set of macros for hand written assembly code
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Hand written assembly codes (.S files) also need paravirtualization
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because they include sensitive instructions or some of code paths in
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them are very performance critical.
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The relation to the IA64 machine vector
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---------------------------------------
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Linux/IA64 has the IA64 machine vector functionality which allows the
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kernel to switch implementations (e.g. initialization, ipi, dma api...)
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depending on executing platform.
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We can replace some implementations very easily defining a new machine
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vector. Thus another approach for virtualization support would be
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enhancing the machine vector functionality.
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But paravirt_ops approach was taken because
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- virtualization support needs wider support than machine vector does.
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e.g. low level instruction paravirtualization. It must be
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initialized very early before platform detection.
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- virtualization support needs more functionality like binary patch.
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Probably the calling overhead might not be very large compared to the
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emulation overhead of virtualization. However in the native case, the
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overhead should be eliminated completely.
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A single kernel binary should run on each environment including native,
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and the overhead of paravirt_ops on native environment should be as
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small as possible.
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- for full virtualization technology, e.g. KVM/IA64 or
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Xen/IA64 HVM domain, the result would be
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(the emulated platform machine vector. probably dig) + (pv_ops).
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This means that the virtualization support layer should be under
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the machine vector layer.
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Possibly it might be better to move some function pointers from
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paravirt_ops to machine vector. In fact, Xen domU case utilizes both
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pv_ops and machine vector.
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IA64 paravirt_ops
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-----------------
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In this section, the concrete paravirt_ops will be discussed.
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Because of the architecture difference between ia64 and x86, the
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resulting set of functions is very different from x86 pv_ops.
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- C function pointer tables
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They are not very performance critical so that simple C indirect
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function call is acceptable. The following structures are defined at
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this moment. For details see linux/include/asm-ia64/paravirt.h
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- struct pv_info
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This structure describes the execution environment.
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- struct pv_init_ops
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This structure describes the various initialization hooks.
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- struct pv_iosapic_ops
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This structure describes hooks to iosapic operations.
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- struct pv_irq_ops
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This structure describes hooks to irq related operations
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- struct pv_time_op
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This structure describes hooks to steal time accounting.
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- a set of indirect calls which need optimization
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Currently this class of functions correspond to a subset of IA64
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intrinsics. At this moment the optimization with binary patch isn't
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implemented yet.
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struct pv_cpu_op is defined. For details see
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linux/include/asm-ia64/paravirt_privop.h
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Mostly they correspond to ia64 intrinsics 1-to-1.
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Caveat: Now they are defined as C indirect function pointers, but in
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order to support binary patch optimization, they will be changed
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using GCC extended inline assembly code.
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- a set of macros for hand written assembly code (.S files)
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For maintenance purpose, the taken approach for .S files is single
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source code and compile multiple times with different macros definitions.
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Each pv_ops instance must define those macros to compile.
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The important thing here is that sensitive, but non-privileged
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instructions must be paravirtualized and that some privileged
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instructions also need paravirtualization for reasonable performance.
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Developers who modify .S files must be aware of that. At this moment
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an easy checker is implemented to detect paravirtualization breakage.
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But it doesn't cover all the cases.
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Sometimes this set of macros is called pv_cpu_asm_op. But there is no
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corresponding structure in the source code.
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Those macros mostly 1:1 correspond to a subset of privileged
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instructions. See linux/include/asm-ia64/native/inst.h.
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And some functions written in assembly also need to be overrided so
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that each pv_ops instance have to define some macros. Again see
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linux/include/asm-ia64/native/inst.h.
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Those structures must be initialized very early before start_kernel.
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Probably initialized in head.S using multi entry point or some other trick.
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For native case implementation see linux/arch/ia64/kernel/paravirt.c.
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