Re: [PATCH] x86/perf: Fix guest_get_msrs static call if there is no PMU

[Peter Zijlstra]

On Mon, Mar 08, 2021 at 10:25:59AM +0800, Xu, Like wrote:
> On 2021/3/6 6:33, Sean Christopherson wrote:
> > Handle a NULL x86_pmu.guest_get_msrs at invocation instead of patching
> > in perf_guest_get_msrs_nop() during setup.  If there is no PMU, setup
> 
> "If there is no PMU" ...

Then you shouldn't be calling this either ofcourse :-)

> > @@ -671,7 +671,11 @@ void x86_pmu_disable_all(void)
> >   struct perf_guest_switch_msr *perf_guest_get_msrs(int *nr)
> >   {
> > -	return static_call(x86_pmu_guest_get_msrs)(nr);
> > +	if (x86_pmu.guest_get_msrs)
> > +		return static_call(x86_pmu_guest_get_msrs)(nr);
> 
> How about using "static_call_cond" per commit "452cddbff7" ?

Given the one user in atomic_switch_perf_msrs() that should work because
it doesn't seem to care about nr_msrs when !msrs.

Still, it calling atomic_switch_perf_msrs() and
intel_pmu_lbr_is_enabled() when there isn't a PMU at all is of course, a
complete waste of cycles.