linux-kernel - Re: [PATCH v6 05/20] KVM: x86/pmu: Allow programming events that match unsupported arch events

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Date:   Sat, 4 Nov 2023 05:46:06 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Kan Liang <kan.liang@...ux.intel.com>,
        Dapeng Mi <dapeng1.mi@...ux.intel.com>,
        Jinrong Liang <cloudliang@...cent.com>,
        Like Xu <likexu@...cent.com>,
        Aaron Lewis <aaronlewis@...gle.com>
Subject: Re: [PATCH v6 05/20] KVM: x86/pmu: Allow programming events that
 match unsupported arch events

On Fri, Nov 3, 2023 at 5:02 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> Remove KVM's bogus restriction that the guest can't program an event whose
> encoding matches an unsupported architectural event.  The enumeration of
> an architectural event only says that if a CPU supports an architectural
> event, then the event can be programmed using the architectural encoding.
> The enumeration does NOT say anything about the encoding when the CPU
> doesn't report support the architectural event.
>
> Preventing the guest from counting events whose encoding happens to match
> an architectural event breaks existing functionality whenever Intel adds
> an architectural encoding that was *ever* used for a CPU that doesn't
> enumerate support for the architectural event, even if the encoding is for
> the exact same event!
>
> E.g. the architectural encoding for Top-Down Slots is 0x01a4.  Broadwell
> CPUs, which do not support the Top-Down Slots architectural event, 0x10a4
> is a valid, model-specific event.  Denying guest usage of 0x01a4 if/when
> KVM adds support for Top-Down slots would break any Broadwell-based guest.
>
> Reported-by: Kan Liang <kan.liang@...ux.intel.com>
> Closes: https://lore.kernel.org/all/2004baa6-b494-462c-a11f-8104ea152c6a@linux.intel.com
> Cc: Dapeng Mi <dapeng1.mi@...ux.intel.com>
> Fixes: a21864486f7e ("KVM: x86/pmu: Fix available_event_types check for REF_CPU_CYCLES event")
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Yes! Finally!

Reviewed-by: Jim Mattson <jmattson@...gle.com>