Re: [PATCH v2] x86/perf: Use RET0 as default for guest_get_msrs to handle "no PMU" case

[Sean Christopherson]

On Tue, Mar 09, 2021, Jim Mattson wrote:
> On Tue, Mar 9, 2021 at 9:10 AM Sean Christopherson <seanjc@xxxxxxxxxx>
> 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.