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diff --git a/Documentation/kvm/ppc-pv.txt b/Documentation/kvm/ppc-pv.txt deleted file mode 100644 index a7f2244b3be..00000000000 --- a/Documentation/kvm/ppc-pv.txt +++ /dev/null @@ -1,196 +0,0 @@ -The PPC KVM paravirtual interface -================================= - -The basic execution principle by which KVM on PowerPC works is to run all kernel -space code in PR=1 which is user space. This way we trap all privileged -instructions and can emulate them accordingly. - -Unfortunately that is also the downfall. There are quite some privileged -instructions that needlessly return us to the hypervisor even though they -could be handled differently. - -This is what the PPC PV interface helps with. It takes privileged instructions -and transforms them into unprivileged ones with some help from the hypervisor. -This cuts down virtualization costs by about 50% on some of my benchmarks. - -The code for that interface can be found in arch/powerpc/kernel/kvm* - -Querying for existence -====================== - -To find out if we're running on KVM or not, we leverage the device tree. When -Linux is running on KVM, a node /hypervisor exists. That node contains a -compatible property with the value "linux,kvm". - -Once you determined you're running under a PV capable KVM, you can now use -hypercalls as described below. - -KVM hypercalls -============== - -Inside the device tree's /hypervisor node there's a property called -'hypercall-instructions'. This property contains at most 4 opcodes that make -up the hypercall. To call a hypercall, just call these instructions. - -The parameters are as follows: - - Register IN OUT - - r0 - volatile - r3 1st parameter Return code - r4 2nd parameter 1st output value - r5 3rd parameter 2nd output value - r6 4th parameter 3rd output value - r7 5th parameter 4th output value - r8 6th parameter 5th output value - r9 7th parameter 6th output value - r10 8th parameter 7th output value - r11 hypercall number 8th output value - r12 - volatile - -Hypercall definitions are shared in generic code, so the same hypercall numbers -apply for x86 and powerpc alike with the exception that each KVM hypercall -also needs to be ORed with the KVM vendor code which is (42 << 16). - -Return codes can be as follows: - - Code Meaning - - 0 Success - 12 Hypercall not implemented - <0 Error - -The magic page -============== - -To enable communication between the hypervisor and guest there is a new shared -page that contains parts of supervisor visible register state. The guest can -map this shared page using the KVM hypercall KVM_HC_PPC_MAP_MAGIC_PAGE. - -With this hypercall issued the guest always gets the magic page mapped at the -desired location in effective and physical address space. For now, we always -map the page to -4096. This way we can access it using absolute load and store -functions. The following instruction reads the first field of the magic page: - - ld rX, -4096(0) - -The interface is designed to be extensible should there be need later to add -additional registers to the magic page. If you add fields to the magic page, -also define a new hypercall feature to indicate that the host can give you more -registers. Only if the host supports the additional features, make use of them. - -The magic page has the following layout as described in -arch/powerpc/include/asm/kvm_para.h: - -struct kvm_vcpu_arch_shared { - __u64 scratch1; - __u64 scratch2; - __u64 scratch3; - __u64 critical; /* Guest may not get interrupts if == r1 */ - __u64 sprg0; - __u64 sprg1; - __u64 sprg2; - __u64 sprg3; - __u64 srr0; - __u64 srr1; - __u64 dar; - __u64 msr; - __u32 dsisr; - __u32 int_pending; /* Tells the guest if we have an interrupt */ -}; - -Additions to the page must only occur at the end. Struct fields are always 32 -or 64 bit aligned, depending on them being 32 or 64 bit wide respectively. - -Magic page features -=================== - -When mapping the magic page using the KVM hypercall KVM_HC_PPC_MAP_MAGIC_PAGE, -a second return value is passed to the guest. This second return value contains -a bitmap of available features inside the magic page. - -The following enhancements to the magic page are currently available: - - KVM_MAGIC_FEAT_SR Maps SR registers r/w in the magic page - -For enhanced features in the magic page, please check for the existence of the -feature before using them! - -MSR bits -======== - -The MSR contains bits that require hypervisor intervention and bits that do -not require direct hypervisor intervention because they only get interpreted -when entering the guest or don't have any impact on the hypervisor's behavior. - -The following bits are safe to be set inside the guest: - - MSR_EE - MSR_RI - MSR_CR - MSR_ME - -If any other bit changes in the MSR, please still use mtmsr(d). - -Patched instructions -==================== - -The "ld" and "std" instructions are transormed to "lwz" and "stw" instructions -respectively on 32 bit systems with an added offset of 4 to accomodate for big -endianness. - -The following is a list of mapping the Linux kernel performs when running as -guest. Implementing any of those mappings is optional, as the instruction traps -also act on the shared page. So calling privileged instructions still works as -before. - -From To -==== == - -mfmsr rX ld rX, magic_page->msr -mfsprg rX, 0 ld rX, magic_page->sprg0 -mfsprg rX, 1 ld rX, magic_page->sprg1 -mfsprg rX, 2 ld rX, magic_page->sprg2 -mfsprg rX, 3 ld rX, magic_page->sprg3 -mfsrr0 rX ld rX, magic_page->srr0 -mfsrr1 rX ld rX, magic_page->srr1 -mfdar rX ld rX, magic_page->dar -mfdsisr rX lwz rX, magic_page->dsisr - -mtmsr rX std rX, magic_page->msr -mtsprg 0, rX std rX, magic_page->sprg0 -mtsprg 1, rX std rX, magic_page->sprg1 -mtsprg 2, rX std rX, magic_page->sprg2 -mtsprg 3, rX std rX, magic_page->sprg3 -mtsrr0 rX std rX, magic_page->srr0 -mtsrr1 rX std rX, magic_page->srr1 -mtdar rX std rX, magic_page->dar -mtdsisr rX stw rX, magic_page->dsisr - -tlbsync nop - -mtmsrd rX, 0 b <special mtmsr section> -mtmsr rX b <special mtmsr section> - -mtmsrd rX, 1 b <special mtmsrd section> - -[Book3S only] -mtsrin rX, rY b <special mtsrin section> - -[BookE only] -wrteei [0|1] b <special wrteei section> - - -Some instructions require more logic to determine what's going on than a load -or store instruction can deliver. To enable patching of those, we keep some -RAM around where we can live translate instructions to. What happens is the -following: - - 1) copy emulation code to memory - 2) patch that code to fit the emulated instruction - 3) patch that code to return to the original pc + 4 - 4) patch the original instruction to branch to the new code - -That way we can inject an arbitrary amount of code as replacement for a single -instruction. This allows us to check for pending interrupts when setting EE=1 -for example. |