On Sun, Nov 14, 2021 at 7:43 PM Like Xu <like.xu.linux@xxxxxxxxx> wrote: > > On 13/11/2021 7:52 am, Jim Mattson wrote: > > Google Cloud has a customer that needs accurate virtualization of two > > architected PMU events on Intel hardware: "instructions retired" and > > "branch instructions retired." The existing PMU virtualization code > > fails to account for instructions that are emulated by kvm. > > Does this customer need to set force_emulation_prefix=Y ? No. That module parameter does make it easier to write the test, though. It's possible that the L0 hypervisor will never emulate a branch instruction for this use case. However, since the code being instrumented is potential malware, one can't make the usual assumptions about "well-behaved" code. For example, it is quite possible that the code in question deliberately runs with the TLBs and in-memory page tables out of sync. Therefore, it's hard to prove that the "branch instructions retired" patch isn't needed. > Is this "accurate statistics" capability fatal to the use case ? Yes, that is my understanding. > > > > Accurately virtualizing all PMU events for all microarchitectures is a > > herculean task, but there are only 8 architected events, so maybe we > > can at least try to get those right. > > I assume you mean the architectural events "Instruction Retired" > and "Branch Instruction Retired" defined by the Intel CPUID > since it looks we don't have a similar concept on AMD. Yes. > This patch set opens Pandora's Box, especially when we have > the real accurate Guest PEBS facility, and things get even > more complicated for just some PMU corner use cases. KVM's PMU virtualization is rife with bugs, but this patch set doesn't make that worse. It actually makes things better by fixing two of those bugs. > > > > Eric Hankland wrote this code originally, but his plate is full, so > > I've volunteered to shepherd the changes through upstream acceptance. > > Does Eric have more code to implement > accurate virtualization on the following events ? No. We only offer PMU virtualization to one customer, and that customer is only interested in the two events addressed by this patch set. > "UnHalted Core Cycles" > "UnHalted Reference Cycles" > "LLC Reference" > "LLC Misses" > "Branch Misses Retired" > "Topdown Slots" (unimplemented) > > Obviously, it's difficult, even absurd, to emulate these. Sorry; I should not have mentioned the eight architected events. It's not entirely clear what some of these events mean in a virtual environment. Let's just stick to the two events covered by this patch set. > > Jim Mattson (2): > > KVM: x86: Update vPMCs when retiring instructions > > KVM: x86: Update vPMCs when retiring branch instructions > > > > arch/x86/kvm/emulate.c | 57 +++++++++++++++++++++----------------- > > arch/x86/kvm/kvm_emulate.h | 1 + > > arch/x86/kvm/pmu.c | 31 +++++++++++++++++++++ > > arch/x86/kvm/pmu.h | 1 + > > arch/x86/kvm/vmx/nested.c | 6 +++- > > arch/x86/kvm/x86.c | 5 ++++ > > 6 files changed, 75 insertions(+), 26 deletions(-) > >
