On Tue, May 16, 2023, Like Xu wrote: > On 27/4/2023 5:53 pm, Anselm Busse wrote: > > Starting with v5, the Intel PMU allows to indicate the available fixed > > function PMCs not only through CPUID.0AH.EDX[4:0] but also through a > > bit mask in CPUID.0AH.ECX. According to the SDM the OS can consider a > > fix function PMC i supported for: > > > > CPUID.0AH.ECX[i] || (CPUID.0AH.EDX[4:0] > i) > > Yes, this feature is attractive for virtualization scenarios, and it gives > flexibility to control which fixed counters are available or not in the > virtual machine. > > However, currently KVM/x86 also supports Intel PMU V2, so I would expect > that we will review the enablement code for v3 and v4 first. Looking at v3, I think we probably want to skip straight to v5. I don't see a sane way for KVM to emulate/virtualize AnyThread, which comes in v3 without a separate CPUID feature flag. The SDM even calls out that it'd be a mess to deal with in a virtualized environment. v5 introduces a CPUID bit to allow deprecating AnyThread, i.e. would give KVM the ability to advertise a sane vPMU model to userspace. Amusingly, KVM advertises "edx.split.anythread_deprecated = 1" for v1+, so maybe we don't even need to do any enabling? At glance, I don't see any other changes in v3 that require KVM support. v4 looks to be an entirely different story than v3 though. So I agree with Like that we need to enable v3 and v4 before advertising support for v5. And KVM *does* need to actually advertise v5. Emulating the fixed counter bitmap without a way to tell userspace about the functionality will create a mess. TL;DR: If y'all want the shiny features in v5, please enable v3 and v4 first. I'm totally fine taking a series to go all the way to v5 (might even be preferred due to the AnyThread crud), but I don't want to advertise v5 without supporting the required v3/v4 features. > Ref: https://lore.kernel.org/kvm/CALMp9eQVnk8gkOpX5AHhaCr8-5Fe=qNuX8PUP1Gv2H5FSYmHSw@xxxxxxxxxxxxxx/ I agree 100% with Jim, the bitmask stuff is firmly v5+.
