linux-kernel - Re: [PATCH v3 1/3] KVM: x86/pmu: Stop adding speculative Intel GP PMCs that don't exist yet

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Message-ID: <Y0CAHch5UR2Lp0tU@google.com>
Date:   Fri, 7 Oct 2022 19:38:05 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH v3 1/3] KVM: x86/pmu: Stop adding speculative Intel GP
 PMCs that don't exist yet

On Mon, Sep 19, 2022, Like Xu wrote:
> From: Like Xu <likexu@...cent.com>
> 
> The Intel April 2022 SDM - Table 2-2. IA-32 Architectural MSRs adds
> a new architectural IA32_OVERCLOCKING_STATUS msr (0x195), plus the
> presence of IA32_CORE_CAPABILITIES (0xCF), the theoretical effective
> maximum value of the Intel GP PMCs is 14 (0xCF - 0xC1) instead of 18.
> 
> But the conclusion of this speculation "14" is very fragile and can
> easily be overturned once Intel declares another meaningful arch msr

s/msr/MSR for consistency

> in the above reserved range, and even worse, just conjecture, Intel
> probably put PMCs 8-15 in a completely different range of MSR indices.
> 
> A conservative proposal would be to stop at the maximum number of Intel
> GP PMCs supported today. Also subsequent changes would limit both AMD
> and Intel on the number of GP counter supported by KVM.
> 
> There are some boxes like Intel P4 (non Architectural PMU) may indeed
> have 18 counters , but those counters are in a completely different msr

unnecessary whitespace before the comma.  And s/msr/MSR again.

> address range and is not supported by KVM.
> 
> Cc: Vitaly Kuznetsov <vkuznets@...hat.com>
> Fixes: cf05a67b68b8 ("KVM: x86: omit "impossible" pmu MSRs from MSR list")

Does this need Cc: stable@...r.kernel.org?  Or is this benign enough that we don't
care?

No need for a v4, the above nits can be handled when applying.

> Suggested-by: Jim Mattson <jmattson@...gle.com>
> Signed-off-by: Like Xu <likexu@...cent.com>
> Reviewed-by: Jim Mattson <jmattson@...gle.com>
> ---

In the future, please provide a cover letter even for trivial series, it helps
(me at least) mentally organize patches.

Thanks!