Re: [PATCH v2 2/6] KVM: x86/mmu: Fix use of enums in trace_fast_page_fault

[Sean Christopherson <seanjc@google.com>]

On Mon, Jul 12, 2021, David Matlack wrote:
> On Mon, Jul 12, 2021 at 09:14:01AM -0700, Ben Gardon wrote:
> > On Wed, Jun 30, 2021 at 2:48 PM David Matlack <dmatlack@google.com> wrote:
> > >
> > > Enum values have to be exported to userspace since the formatting is not
> > > done in the kernel. Without doing this perf maps RET_PF_FIXED and
> > > RET_PF_SPURIOUS to 0, which results in incorrect output:

Oof, that's brutal.

> > >   $ perf record -a -e kvmmmu:fast_page_fault --filter "ret==3" -- ./access_tracking_perf_test
> > >   $ perf script | head -1
> > >    [...] new 610006048d25877 spurious 0 fixed 0  <------ should be 1
> > >
> > > Fix this by exporting the enum values to userspace with TRACE_DEFINE_ENUM.
> > >
> > > Fixes: c4371c2a682e ("KVM: x86/mmu: Return unique RET_PF_* values if the fault was fixed")
> > > Signed-off-by: David Matlack <dmatlack@google.com>
> > > ---
> > >  arch/x86/kvm/mmu/mmutrace.h | 3 +++
> > >  1 file changed, 3 insertions(+)
> > >
> > > diff --git a/arch/x86/kvm/mmu/mmutrace.h b/arch/x86/kvm/mmu/mmutrace.h
> > > index efbad33a0645..55c7e0fcda52 100644
> > > --- a/arch/x86/kvm/mmu/mmutrace.h
> > > +++ b/arch/x86/kvm/mmu/mmutrace.h
> > > @@ -244,6 +244,9 @@ TRACE_EVENT(
> > >                   __entry->access)
> > >  );
> > >
> > > +TRACE_DEFINE_ENUM(RET_PF_FIXED);
> > > +TRACE_DEFINE_ENUM(RET_PF_SPURIOUS);
> > > +
> > 
> > If you're planning to send out a v3 anyway, it might be worth adding
> > all the PF return code enums:
> > 
> > enum {
> > RET_PF_RETRY = 0,
> > RET_PF_EMULATE,
> > RET_PF_INVALID,
> > RET_PF_FIXED,
> > RET_PF_SPURIOUS,
> > };
> > 
> > Just so that no one has to worry about this in the future.

Until someone adds a new enum :-/

> Will do in v3. Thanks.

What about converting the enums to #defines, with a blurb in the comment
explaining that the values are arbitrary but aren't enums purely to avoid this
tracepoint issue?