linux-kernel - Re: [PATCH v2] x86/perf: Use RET0 as default for guest_get

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Message-ID: <YEfQir8rv8/j2hgn@google.com>
Date:   Tue, 9 Mar 2021 11:46:18 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        the arch/x86 maintainers <x86@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        Like Xu <like.xu@...ux.intel.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        syzbot+cce9ef2dd25246f815ee@...kaller.appspotmail.com
Subject: Re: [PATCH v2] x86/perf: Use RET0 as default for guest_get_msrs to
 handle "no PMU" case

On Tue, Mar 09, 2021, Jim Mattson wrote:
> On Tue, Mar 9, 2021 at 9:10 AM Sean Christopherson <seanjc@...gle.com>
> wrote:
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index 50810d471462..32cf8287d4a7 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -6580,8 +6580,8 @@ static void atomic_switch_perf_msrs(struct vcpu_vmx
> > *vmx)
> >         int i, nr_msrs;
> >         struct perf_guest_switch_msr *msrs;
> >
> > +       /* Note, nr_msrs may be garbage if perf_guest_get_msrs() returns
> > NULL. */
> >
> 
> You could drop the scary comment with a profligate initialization of
> nr_msrs to 0.

Yeah, I considered that as well.  I opted for the scary comment because I
wanted to dissuade future patches from modifying this code without taking into
account the non-obvious behavior.