linux-kernel - Re: [PATCH] KVM: vPMU: Use atomic bit operations for global

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Message-ID: <CAL715WJwb8RhqdvOaurUxY5=ftixw7LHZWBT9PrOGRAzF5veqA@mail.gmail.com>
Date:   Mon, 11 Sep 2023 16:42:46 -0700
From:   Mingwei Zhang <mizhang@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Dapeng Mi <dapeng1.mi@...el.com>,
        Jim Mattson <jmattson@...gle.com>, Like Xu <likexu@...cent.com>
Subject: Re: [PATCH] KVM: vPMU: Use atomic bit operations for global_status

On Mon, Sep 11, 2023 at 11:09 AM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Mon, Sep 11, 2023, Mingwei Zhang wrote:
> > On Mon, Sep 11, 2023 at 8:01 AM Sean Christopherson <seanjc@...gle.com> wrote:
> > >
> > > On Mon, Sep 11, 2023, Mingwei Zhang wrote:
> > > > Use atomic bit operations for pmu->global_status because it may suffer from
> > > > race conditions between emulated overflow in KVM vPMU and PEBS overflow in
> > > > host PMI handler.
> > >
> > > Only if the host PMI occurs on a different pCPU, and if that can happen don't we
> > > have a much larger problem?
> >
> > Why on different pCPU?  For vPMU, I think there is always contention
> > between the vCPU thread and the host PMI handler running on the same
> > pCPU, no?
>
> A non-atomic instruction can't be interrupted by an NMI, or any other event, so
> I don't see how switching to atomic operations fixes anything unless the NMI comes
> in on a different pCPU.

You are right. I realize that. The race condition has to happen
concurrently from two different pCPUs. This happens to
pmu->reprogram_nmi but not pmu->global_status. The critical stuff we
care about should be re-entrancy issues for __kvm_perf_overflow() and
some state maintenance issues like avoiding duplicate NMI injection.

That concludes that we don't need this change.

Thanks
-Mingwei