On Tue, Jan 31, 2023, Like Xu wrote:
> On 28/1/2023 8:14 am, Sean Christopherson wrote:
> > Disallow enabling LBR support if the CPU supports architectural LBRs.
> > Traditional LBR support is absent on CPU models that have architectural
> > LBRs, and KVM doesn't yet support arch LBRs, i.e. KVM will pass through
> > non-existent MSRs if userspace enables LBRs for the guest.
>
> True, we have call_trace due to MSR_ARCH_LBR_FROM_0 (0x1500) for example.
>
> >
> > Cc: stable@xxxxxxxxxxxxxxx
> > Cc: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> > Cc: Like Xu <like.xu.linux@xxxxxxxxx>
>
> Tested-by: Like Xu <likexu@xxxxxxxxxxx>
>
> > Reported-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>
> Fixes: 145dfad998ea ("KVM: VMX: Advertise PMU LBRs if and only if perf
> supports LBRs")
If we want a fixes, I'd argue this is more appropriate:
Fixes: be635e34c284 ("KVM: vmx/pmu: Expose LBR_FMT in the MSR_IA32_PERF_CAPABILITIES")
Though I'd prefer not to blame KVM, there's not much we could have done in KVM
to know that Intel would effectively break backwards compatibility.
> > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > ---
> >
> > Am I missing something that would prevent this scenario?
> >
> > arch/x86/kvm/vmx/vmx.c | 8 +++++---
> > 1 file changed, 5 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index 8f0f67c75f35..77ee6b4a5ec4 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -7761,9 +7761,11 @@ static u64 vmx_get_perf_capabilities(void)
> > if (boot_cpu_has(X86_FEATURE_PDCM))
> > rdmsrl(MSR_IA32_PERF_CAPABILITIES, host_perf_cap);
> > - x86_perf_get_lbr(&lbr);
> > - if (lbr.nr)
> > - perf_cap |= host_perf_cap & PMU_CAP_LBR_FMT;
> > + if (!cpu_feature_enabled(X86_FEATURE_ARCH_LBR)) {
>
> To avoid changing this again in the Arch lbr enabling part, how about:
>
> x86_perf_get_lbr(&lbr);
> if (lbr.nr && cpu_feature_enabled(X86_FEATURE_ARCH_LBR) ==
> kvm_cpu_cap_has(X86_FEATURE_ARCH_LBR))
> perf_cap |= host_perf_cap & PMU_CAP_LBR_FMT;
>
> ?
I'd rather force arch LBR enabling to explicitly update this code. And I'd prefer
that KVM explicitly clear PMU_CAP_LBR_FMT when KVM can't use arch LBRs for whatever
reason, both for documentation purposes and to avoid ordering dependencies between
consuming vmx_get_perf_capabilities() and updating kvm_cpu_cap_has(X86_FEATURE_ARCH_LBR).
