On Mon, 2024-07-08 at 17:24 -0700, Sean Christopherson wrote: > On Thu, Jul 04, 2024, Maxim Levitsky wrote: > > On Fri, 2024-05-17 at 10:39 -0700, Sean Christopherson wrote: > > > - cpuid_entry_change(best, X86_FEATURE_OSPKE, > > > - kvm_is_cr4_bit_set(vcpu, X86_CR4_PKE)); > > > + kvm_update_feature_runtime(vcpu, best, X86_FEATURE_OSPKE, > > > + kvm_is_cr4_bit_set(vcpu, X86_CR4_PKE)); > > > + > > > > > > best = kvm_find_cpuid_entry_index(vcpu, 0xD, 0); > > > if (best) > > > > I am not 100% sure that we need to do this. > > > > Runtime cpuid changes are a hack that Intel did back then, due to various > > reasons, These changes don't really change the feature set that CPU supports, > > but merly as you like to say 'massage' the output of the CPUID instruction to > > make the unmodified OS happy usually. > > > > Thus it feels to me that CPU caps should not include the dynamic features, > > and neither KVM should use the value of these as a source for truth, but > > rather the underlying source of the truth (e.g CR4). > > > > But if you insist, I don't really have a very strong reason to object this. > > FWIW, I think I agree that CR4 should be the source of truth, but it's largely a > moot point because KVM doesn't actually check OSXSAVE or OSPKE, as KVM never > emulates the relevant instructions. So for those, it's indeed not strictly > necessary. > > Unfortunately, KVM has established ABI for checking X86_FEATURE_MWAIT when > "emulating" MONITOR and MWAIT, i.e. KVM can't use vcpu->arch.ia32_misc_enable_msr > as the source of truth. Can you elaborate on this? Can you give me an example of the ABI? > So for MWAIT, KVM does need to update CPU caps (or carry > even more awful MWAIT code), at which point extending the behavior to the CR4 > features (and to X86_FEATURE_APIC) is practically free. > Best regards, Maxim Levitsky
