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

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Date:   Fri, 3 Feb 2023 21:03:31 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Yang Weijiang <weijiang.yang@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH] KVM: x86/pmu: Disallow legacy LBRs if architectural LBRs
 are available

On Fri, Feb 03, 2023, Like Xu wrote:
> On 3/2/2023 3:11 am, Sean Christopherson wrote:
> > 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@...r.kernel.org
> > > > Cc: Yang Weijiang <weijiang.yang@...el.com>
> > > > Cc: Like Xu <like.xu.linux@...il.com>
> > > 
> > > Tested-by: Like Xu <likexu@...cent.com>
> > > 
> > > > Reported-by: Paolo Bonzini <pbonzini@...hat.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.
> 
> Personally, I assume the bigger role of the Fix tag is to help the stable tree's
> bots make it easier to back port patches automatically, and there will be less
> sense of blame for the developers.

I don't mind adding a Fixes to aid stable, but then

  Fixes: be635e34c284 ("KVM: vmx/pmu: Expose LBR_FMT in the MSR_IA32_PERF_CAPABILITIES")

is still more correct, e.g. if there are kernel's that didn't get
145dfad998ea ("KVM: VMX: Advertise PMU LBRs if and only if perf supports LBRs")
for whatever reason.

> In pmu scope, if a feature is not "architecture", I'm not surprised that a
> new arrival will break compatibility, and sometimes kernel developers need to
> plan ahead.

Hrm, true, compatibility is usually a non-goal for uarch stuff.  I'll try to keep
that in mind for future vPMU code.

Thanks!