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[Xen-devel] [PATCH] virtual: Documentation: simplify and generalize paravirt_ops.txt

From: "Luis R. Rodriguez" <mcgrof@xxxxxxxx> The general documentation we have for pv_ops is currenty present on the IA64 docs, but since this documentation covers IA64 xen enablement and IA64 Xen support got ripped out a while ago through commit d52eefb47 present since v3.14-rc1 lets just simplify, generalize and move the pv_ops documentation to a shared place. Cc: Isaku Yamahata <yamahata@xxxxxxxxxxxxx> Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx> Cc: Chris Wright <chrisw@xxxxxxxxxxxx> Cc: Alok Kataria <akataria@xxxxxxxxxx> Cc: Rusty Russell <rusty@xxxxxxxxxxxxxxx> Cc: virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx Cc: Tony Luck <tony.luck@xxxxxxxxx> Cc: Fenghua Yu <fenghua.yu@xxxxxxxxx> Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx Cc: kvm@xxxxxxxxxxxxxxx Cc: linux-kernel@xxxxxxxxxxxxxxx Signed-off-by: Luis R. Rodriguez <mcgrof@xxxxxxxx> --- Documentation/ia64/paravirt_ops.txt | 137 --------------------------------- Documentation/virtual/00-INDEX | 3 + Documentation/virtual/paravirt_ops.txt | 32 ++++++++ MAINTAINERS | 2 +- 4 files changed, 36 insertions(+), 138 deletions(-) delete mode 100644 Documentation/ia64/paravirt_ops.txt create mode 100644 Documentation/virtual/paravirt_ops.txt diff --git a/Documentation/ia64/paravirt_ops.txt b/Documentation/ia64/paravirt_ops.txt deleted file mode 100644 index 39ded02..0000000 --- a/Documentation/ia64/paravirt_ops.txt +++ /dev/null @@ -1,137 +0,0 @@ -Paravirt_ops on IA64 -==================== - 21 May 2008, Isaku Yamahata <yamahata@xxxxxxxxxxxxx> - - -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. diff --git a/Documentation/virtual/00-INDEX b/Documentation/virtual/00-INDEX index e952d30..af0d239 100644 --- a/Documentation/virtual/00-INDEX +++ b/Documentation/virtual/00-INDEX @@ -2,6 +2,9 @@ Virtualization support in the Linux kernel. 00-INDEX - this file. + +paravirt_ops.txt + - Describes the Linux kernel pv_ops to support different hypervisors kvm/ - Kernel Virtual Machine. See also http://linux-kvm.org uml/ diff --git a/Documentation/virtual/paravirt_ops.txt b/Documentation/virtual/paravirt_ops.txt new file mode 100644 index 0000000..d4881c0 --- /dev/null +++ b/Documentation/virtual/paravirt_ops.txt @@ -0,0 +1,32 @@ +Paravirt_ops +============ + +Linux provides support for different hypervisor virtualization technologies. +Historically different binary kernels would be required in order to support +different hypervisors, this restriction was removed with pv_ops. +Linux pv_ops is a virtualization API which enables support for different +hypervisors. It allows each hypervisor to override critical operations and +allows a single kernel binary to run on all supported execution environments +including native machine -- without any hypervisors. + +pv_ops provides a set of function pointers which represent operations +corresponding to low level critical instructions and high level +functionalities in various areas. pv-ops allows for optimizations at run +time by enabling binary patching of the low-ops critical operations +at boot time. + +pv_ops operations are classified into three categories: + +- simple indirect call + These operations correspond to high level functionality where it is + known 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 critical instructions. They + are called frequently and are performance critical. 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. diff --git a/MAINTAINERS b/MAINTAINERS index cee31c9..0df4446 100644 --- a/MAINTAINERS +++ b/MAINTAINERS @@ -7267,7 +7267,7 @@ M: Alok Kataria <akataria@xxxxxxxxxx> M: Rusty Russell <rusty@xxxxxxxxxxxxxxx> L: virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx S: Supported -F: Documentation/ia64/paravirt_ops.txt +F: Documentation/virtual/paravirt_ops.txt F: arch/*/kernel/paravirt* F: arch/*/include/asm/paravirt.h -- 2.2.2 _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel

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