Re: [PATCH v4 05/38] perf: Add generic exclude_guest support

[Sean Christopherson]

On Fri, Apr 25, 2025, Peter Zijlstra wrote:
> On Mon, Mar 24, 2025 at 05:30:45PM +0000, Mingwei Zhang wrote:
> 
> > @@ -6040,6 +6041,71 @@ void perf_put_mediated_pmu(void)
> >  }
> >  EXPORT_SYMBOL_GPL(perf_put_mediated_pmu);
> >  
> > +static inline void perf_host_exit(struct perf_cpu_context *cpuctx)
> > +{
> > +   perf_ctx_disable(&cpuctx->ctx, EVENT_GUEST);
> > +   ctx_sched_out(&cpuctx->ctx, NULL, EVENT_GUEST);
> > +   perf_ctx_enable(&cpuctx->ctx, EVENT_GUEST);
> > +   if (cpuctx->task_ctx) {
> > +           perf_ctx_disable(cpuctx->task_ctx, EVENT_GUEST);
> > +           task_ctx_sched_out(cpuctx->task_ctx, NULL, EVENT_GUEST);
> > +           perf_ctx_enable(cpuctx->task_ctx, EVENT_GUEST);
> > +   }
> > +}
> > +
> > +/* When entering a guest, schedule out all exclude_guest events. */
> > +void perf_guest_enter(void)
> > +{
> > +   struct perf_cpu_context *cpuctx = this_cpu_ptr(&perf_cpu_context);
> > +
> > +   lockdep_assert_irqs_disabled();
> > +
> > +   perf_ctx_lock(cpuctx, cpuctx->task_ctx);
> > +
> > +   if (WARN_ON_ONCE(__this_cpu_read(perf_in_guest)))
> > +           goto unlock;
> > +
> > +   perf_host_exit(cpuctx);
> > +
> > +   __this_cpu_write(perf_in_guest, true);
> > +
> > +unlock:
> > +   perf_ctx_unlock(cpuctx, cpuctx->task_ctx);
> > +}
> > +EXPORT_SYMBOL_GPL(perf_guest_enter);
> > +
> > +static inline void perf_host_enter(struct perf_cpu_context *cpuctx)
> > +{
> > +   perf_ctx_disable(&cpuctx->ctx, EVENT_GUEST);
> > +   if (cpuctx->task_ctx)
> > +           perf_ctx_disable(cpuctx->task_ctx, EVENT_GUEST);
> > +
> > +   perf_event_sched_in(cpuctx, cpuctx->task_ctx, NULL, EVENT_GUEST);
> > +
> > +   if (cpuctx->task_ctx)
> > +           perf_ctx_enable(cpuctx->task_ctx, EVENT_GUEST);
> > +   perf_ctx_enable(&cpuctx->ctx, EVENT_GUEST);
> > +}
> > +
> > +void perf_guest_exit(void)
> > +{
> > +   struct perf_cpu_context *cpuctx = this_cpu_ptr(&perf_cpu_context);
> > +
> > +   lockdep_assert_irqs_disabled();
> > +
> > +   perf_ctx_lock(cpuctx, cpuctx->task_ctx);
> > +
> > +   if (WARN_ON_ONCE(!__this_cpu_read(perf_in_guest)))
> > +           goto unlock;
> > +
> > +   perf_host_enter(cpuctx);
> > +
> > +   __this_cpu_write(perf_in_guest, false);
> > +unlock:
> > +   perf_ctx_unlock(cpuctx, cpuctx->task_ctx);
> > +}
> > +EXPORT_SYMBOL_GPL(perf_guest_exit);
> 
> This naming is confusing on purpose? Pick either guest/host and stick
> with it.

+1.  I also think the inner perf_host_{enter,exit}() helpers are superflous.
These flows

After a bit of hacking, and with a few spoilers, this is what I ended up with
(not anywhere near fully tested).  I like following KVM's kvm_xxx_{load,put}()
nomenclature to tie everything together, so I went with "guest" instead of 
"host"
even though the majority of work being down is to shedule out/in host context.

/* When loading a guest's mediated PMU, schedule out all exclude_guest events. 
*/
void perf_load_guest_context(unsigned long data)
{
        struct perf_cpu_context *cpuctx = this_cpu_ptr(&perf_cpu_context);

        lockdep_assert_irqs_disabled();

        perf_ctx_lock(cpuctx, cpuctx->task_ctx);

        if (WARN_ON_ONCE(__this_cpu_read(guest_ctx_loaded)))
                goto unlock;

        perf_ctx_disable(&cpuctx->ctx, EVENT_GUEST);
        ctx_sched_out(&cpuctx->ctx, NULL, EVENT_GUEST);
        perf_ctx_enable(&cpuctx->ctx, EVENT_GUEST);
        if (cpuctx->task_ctx) {
                perf_ctx_disable(cpuctx->task_ctx, EVENT_GUEST);
                task_ctx_sched_out(cpuctx->task_ctx, NULL, EVENT_GUEST);
                perf_ctx_enable(cpuctx->task_ctx, EVENT_GUEST);
        }

        arch_perf_load_guest_context(data);

        __this_cpu_write(guest_ctx_loaded, true);

unlock:
        perf_ctx_unlock(cpuctx, cpuctx->task_ctx);
}
EXPORT_SYMBOL_GPL(perf_load_guest_context);

void perf_put_guest_context(void)
{
        struct perf_cpu_context *cpuctx = this_cpu_ptr(&perf_cpu_context);

        lockdep_assert_irqs_disabled();

        perf_ctx_lock(cpuctx, cpuctx->task_ctx);

        if (WARN_ON_ONCE(!__this_cpu_read(guest_ctx_loaded)))
                goto unlock;

        arch_perf_put_guest_context();

        perf_ctx_disable(&cpuctx->ctx, EVENT_GUEST);
        if (cpuctx->task_ctx)
                perf_ctx_disable(cpuctx->task_ctx, EVENT_GUEST);

        perf_event_sched_in(cpuctx, cpuctx->task_ctx, NULL, EVENT_GUEST);

        if (cpuctx->task_ctx)
                perf_ctx_enable(cpuctx->task_ctx, EVENT_GUEST);
        perf_ctx_enable(&cpuctx->ctx, EVENT_GUEST);

        __this_cpu_write(guest_ctx_loaded, false);
unlock:
        perf_ctx_unlock(cpuctx, cpuctx->task_ctx);
}
EXPORT_SYMBOL_GPL(perf_put_guest_context);