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

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Mar 09, 2021 at 08:46:49AM +0100, Peter Zijlstra wrote:
> On Mon, Mar 08, 2021 at 12:40:44PM -0800, Sean Christopherson wrote:
> > On Mon, Mar 08, 2021, Peter Zijlstra wrote:
> 
> > > Given the one user in atomic_switch_perf_msrs() that should work because
> > > it doesn't seem to care about nr_msrs when !msrs.
> > 
> > Uh, that commit quite cleary says:
> 
> D0h! I got static_call_cond() and __static_call_return0 mixed up.
> Anyway, let me see if I can make something work here.

Does this work? I can never seem to start a VM, and if I do accidentally
manage, then it never contains the things I need :/

---
diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index 6ddeed3cd2ac..fadcecd73e1a 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -81,7 +81,11 @@ DEFINE_STATIC_CALL_NULL(x86_pmu_swap_task_ctx, *x86_pmu.swap_task_ctx);
 DEFINE_STATIC_CALL_NULL(x86_pmu_drain_pebs,   *x86_pmu.drain_pebs);
 DEFINE_STATIC_CALL_NULL(x86_pmu_pebs_aliases, *x86_pmu.pebs_aliases);
 
-DEFINE_STATIC_CALL_NULL(x86_pmu_guest_get_msrs,  *x86_pmu.guest_get_msrs);
+/*
+ * This one is magic, it will get called even when PMU init fails (because
+ * there is no PMU), in which case it should simply return NULL.
+ */
+__DEFINE_STATIC_CALL(x86_pmu_guest_get_msrs, *x86_pmu.guest_get_msrs, __static_call_return0);
 
 u64 __read_mostly hw_cache_event_ids
 				[PERF_COUNT_HW_CACHE_MAX]
@@ -1944,13 +1948,6 @@ static void _x86_pmu_read(struct perf_event *event)
 	x86_perf_event_update(event);
 }
 
-static inline struct perf_guest_switch_msr *
-perf_guest_get_msrs_nop(int *nr)
-{
-	*nr = 0;
-	return NULL;
-}
-
 static int __init init_hw_perf_events(void)
 {
 	struct x86_pmu_quirk *quirk;
@@ -2024,9 +2021,6 @@ static int __init init_hw_perf_events(void)
 	if (!x86_pmu.read)
 		x86_pmu.read = _x86_pmu_read;
 
-	if (!x86_pmu.guest_get_msrs)
-		x86_pmu.guest_get_msrs = perf_guest_get_msrs_nop;
-
 	x86_pmu_static_call_update();
 
 	/*