Re: [PATCH] KVM: x86/pmu: Disallow legacy LBRs if architectural LBRs are available

[Sean Christopherson <seanjc@google.com>]

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@vger.kernel.org
> > Cc: Yang Weijiang <weijiang.yang@intel.com>
> > Cc: Like Xu <like.xu.linux@gmail.com>
> 
> Tested-by: Like Xu <likexu@tencent.com>
> 
> > Reported-by: Paolo Bonzini <pbonzini@redhat.com>
> 
> 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@google.com>
> > ---
> > 
> > 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).