Re: [RFC PATCH 06/41] perf: x86: Add function to switch PMI handler

[Sean Christopherson]

On Thu, Apr 11, 2024, Sean Christopherson wrote:
> On Fri, Jan 26, 2024, Xiong Zhang wrote:
> > From: Xiong Zhang <xiong.y.zhang@xxxxxxxxx>
> > 
> > Add function to switch PMI handler since passthrough PMU and host PMU will
> > use different interrupt vectors.
> > 
> > Signed-off-by: Xiong Zhang <xiong.y.zhang@xxxxxxxxx>
> > Signed-off-by: Mingwei Zhang <mizhang@xxxxxxxxxx>
> > ---
> >  arch/x86/events/core.c            | 15 +++++++++++++++
> >  arch/x86/include/asm/perf_event.h |  3 +++
> >  2 files changed, 18 insertions(+)
> > 
> > diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> > index 40ad1425ffa2..3f87894d8c8e 100644
> > --- a/arch/x86/events/core.c
> > +++ b/arch/x86/events/core.c
> > @@ -701,6 +701,21 @@ struct perf_guest_switch_msr *perf_guest_get_msrs(int *nr, void *data)
> >  }
> >  EXPORT_SYMBOL_GPL(perf_guest_get_msrs);
> >  
> > +void perf_guest_switch_to_host_pmi_vector(void)
> > +{
> > +	lockdep_assert_irqs_disabled();
> > +
> > +	apic_write(APIC_LVTPC, APIC_DM_NMI);
> > +}
> > +EXPORT_SYMBOL_GPL(perf_guest_switch_to_host_pmi_vector);
> > +
> > +void perf_guest_switch_to_kvm_pmi_vector(void)
> > +{
> > +	lockdep_assert_irqs_disabled();
> > +
> > +	apic_write(APIC_LVTPC, APIC_DM_FIXED | KVM_VPMU_VECTOR);
> > +}
> > +EXPORT_SYMBOL_GPL(perf_guest_switch_to_kvm_pmi_vector);
> 
> Why slice and dice the context switch if it's all in perf?  Just do this in
> perf_guest_enter().  

Ah, because perf_guest_enter() isn't x86-specific.

That can be solved by having the exported APIs be arch specific, e.g.
x86_perf_guest_enter(), and making perf_guest_enter() a perf-internal API.

That has the advantage of making it impossible to call perf_guest_enter() on an
unsupported architecture (modulo perf bugs).