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