linux-kernel - Re: [PATCH v2] KVM: x86/pmu: Make top-down.slots event unavailable in supported leaf

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Date:   Thu, 6 Jan 2022 12:12:23 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Like Xu <like.xu.linux@...il.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] KVM: x86/pmu: Make top-down.slots event unavailable in
 supported leaf

On Thu, Jan 6, 2022 at 10:09 AM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Thu, Jan 06, 2022, Like Xu wrote:
> > From: Like Xu <likexu@...cent.com>
> >
> > When we choose to disable the fourth fixed counter TOPDOWN.SLOTS,
> > we also need to comply with the specification and set 0AH.EBX.[bit 7]
> > to 1 if the guest (e.g. on the ICX) has a value of 0AH.EAX[31:24] > 7.
> >
> > Fixes: 2e8cd7a3b8287 ("kvm: x86: limit the maximum number of vPMU fixed counters to 3")
> > Signed-off-by: Like Xu <likexu@...cent.com>
> > ---
> > v1 -> v2 Changelog:
> > - Make it simpler to keep forward compatibility; (Sean)
> > - Wrap related comment at ~80 chars; (Sean)
> >
> > Previous:
> > https://lore.kernel.org/kvm/20220105050711.67280-1-likexu@tencent.com/
> >
> >  arch/x86/kvm/cpuid.c | 12 ++++++++++++
> >  1 file changed, 12 insertions(+)
> >
> > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > index 0b920e12bb6d..4fe17a537084 100644
> > --- a/arch/x86/kvm/cpuid.c
> > +++ b/arch/x86/kvm/cpuid.c
> > @@ -782,6 +782,18 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
> >               eax.split.mask_length = cap.events_mask_len;
> >
> >               edx.split.num_counters_fixed = min(cap.num_counters_fixed, MAX_FIXED_COUNTERS);
> > +
> > +             /*
> > +              * The 8th Intel architectural event (Topdown Slots) will be supported
>
> Nit, the "8th" part is unnecessary information.
>
> > +              * if the 4th fixed counter exists && EAX[31:24] > 7 && EBX[7] = 0.
> > +              *
> > +              * Currently, KVM needs to set EAX[31:24] < 8 or EBX[7] == 1
> > +              * to make this event unavailable in a consistent way.
> > +              */
>
> This comment is now slightly stale.  It also doesn't say why the event is made
> unavailable.
>
> > +             if (edx.split.num_counters_fixed < 4 &&
>
> Rereading the changelog and the changelog of the Fixed commit, I don't think KVM
> should bother checking num_counters_fixed.  IIUC, cap.events_mask[7] should already
> be '1' if there are less than 4 fixed counters in hardware, but at the same time
> there's no harm in being a bit overzealous.  That would help simplifiy the comment
> as there's no need to explain why num_counters_fixed is checked, e.g. the fact that
> Topdown Slots uses the 4th fixed counter is irrelevant with respect to the legality
> of setting EBX[7]=1 to hide an unsupported event.

I was under the impression that the CPUID.0AH:EBX bits were
independent of the fixed counters. That is, if CPUID.0AH:EAX[31:24] >
7 and CPUID.0AH:EBX[7] is clear, then one should be able to program a
general purpose counter with event selector A4H and umask 01H,
regardless of whether or not fixed counter 4 exists.

>
>                 /*
>                  * Hide Intel's Topdown Slots architectural event, it's not yet
>                  * supported by KVM.
>                  */
>                 if (eax.split.mask_length > 7)
>                         cap.events_mask |= BIT_ULL(7);
>
> > +                 eax.split.mask_length > 7)
> > +                     cap.events_mask |= BIT_ULL(7);
> > +
> >               edx.split.bit_width_fixed = cap.bit_width_fixed;
> >               if (cap.version)
> >                       edx.split.anythread_deprecated = 1;
> > --
> > 2.33.1
> >