linux-kernel - Re: [PATCH] KVM: x86: vPMU: truncate counter value to allowed width

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Date:   Thu, 28 Sep 2023 09:41:56 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>, kvm@...r.kernel.org,
        Roman Kagan <rkagan@...zon.de>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Jim Mattson <jmattson@...gle.com>,
        Like Xu <likexu@...cent.com>,
        Paolo Bonzini <pbonzini@...hat.com>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Eric Hankland <ehankland@...gle.com>,
        linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] KVM: x86: vPMU: truncate counter value to allowed width

On Thu, 04 May 2023 14:00:42 +0200, Roman Kagan wrote:
> Performance counters are defined to have width less than 64 bits.  The
> vPMU code maintains the counters in u64 variables but assumes the value
> to fit within the defined width.  However, for Intel non-full-width
> counters (MSR_IA32_PERFCTRx) the value receieved from the guest is
> truncated to 32 bits and then sign-extended to full 64 bits.  If a
> negative value is set, it's sign-extended to 64 bits, but then in
> kvm_pmu_incr_counter() it's incremented, truncated, and compared to the
> previous value for overflow detection.
> That previous value is not truncated, so it always evaluates bigger than
> the truncated new one, and a PMI is injected.  If the PMI handler writes
> a negative counter value itself, the vCPU never quits the PMI loop.
> 
> [...]

Applied to kvm-x86 pmu, with a slightly massaged changelog.  Thanks!  And sorry
for the horrendous delay...

[1/1] KVM: x86/pmu: Truncate counter value to allowed width on write
      https://github.com/kvm-x86/linux/commit/b29a2acd36dd

--
https://github.com/kvm-x86/linux/tree/next