linux-kernel - Re: [PATCH] KVM: x86/intr: Explicitly check NMI from guest to eliminate false positives

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Date:   Mon, 4 Dec 2023 07:55:45 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/intr: Explicitly check NMI from guest to
 eliminate false positives

On Mon, Dec 04, 2023, Like Xu wrote:
> From: Like Xu <likexu@...cent.com>
> 
> Explicitly checking the source of external interrupt is indeed NMI and not
> other types in the kvm_arch_pmi_in_guest(), which prevents perf-kvm false
> positive samples generated after vm-exit but before kvm_before_interrupt()
> from being incorrectly labelled as guest samples:

...

> Fixes: 73cd107b9685 ("KVM: x86: Drop current_vcpu for kvm_running_vcpu + kvm_arch_vcpu variable")

The behavior is deliberate, and was added by commit dd60d217062f ("KVM: x86: Fix
perf timer mode IP reporting").  *If* we want to undo that, then the best "fix"
would be to effective reverting that commit by dropping the IRQ usage of
kvm_before_interrupt() and renaming the helpers kvm_{before,after}_nmi().  But
my understanding is that the behavior is necessary for select PMU usage.