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diff --git a/Documentation/ia64/paravirt_ops.txt b/Documentation/ia64/paravirt_ops.txt new file mode 100644 index 00000000..39ded02e --- /dev/null +++ b/Documentation/ia64/paravirt_ops.txt @@ -0,0 +1,137 @@ +Paravirt_ops on IA64 +==================== + 21 May 2008, Isaku Yamahata <yamahata@valinux.co.jp> + + +Introduction +------------ +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. + + +IA64 paravirt_ops +----------------- +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 +implemented yet. +struct pv_cpu_op is defined. For details see +linux/include/asm-ia64/paravirt_privop.h +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 +linux/include/asm-ia64/native/inst.h. + + +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. |