Re: [PATCH v3 1/5] KVM: x86/pmu: Prevent the PMU from counting disallowed events

[Sean Christopherson <seanjc@google.com>]

On Tue, Mar 07, 2023, Aaron Lewis wrote:
> On Tue, Mar 7, 2023 at 7:19 AM Like Xu <like.xu.linux@gmail.com> wrote:
> > > ---
> > >   arch/x86/kvm/pmu.c | 13 ++++++++-----
> > >   1 file changed, 8 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> > > index 612e6c70ce2e..9914a9027c60 100644
> > > --- a/arch/x86/kvm/pmu.c
> > > +++ b/arch/x86/kvm/pmu.c
> > > @@ -400,6 +400,12 @@ static bool check_pmu_event_filter(struct kvm_pmc *pmc)
> > >       return is_fixed_event_allowed(filter, pmc->idx);
> > >   }
> > >
> > > +static bool event_is_allowed(struct kvm_pmc *pmc)
> >
> > Nit, an inline event_is_allowed() here might be better.
> 
> I purposely didn't inline this because Sean generally discourages its
> use and has commented in several reviews to not use 'inline' and
> instead leave it up to the compiler to decide, unless using
> __always_inline.

Ya.

> I think the sentiment is either use the strong hint or don't use it at all.
> This seems like an example of where the compiler can decide, and a strong
> hint isn't needed.

Not quite.  __always_inline is not a hint, it's a command.  The kernel *requires*
functions tagged with __always_inline to be (surprise!) always inlined, even when
building with features that cause the compiler to generate non-inlined functions
for even the most trivial helpers, e.g. KASAN can cause a literal nop function to
be non-inlined.  __alway_inlined is used to ensure like no-instrumentation regions
and __init sections are preserved when invoking common helpers.