Re: [PATCH v2] x86/perf: Use RET0 as default for guest_get_msrs to handle "no PMU" case

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Mar 09, 2021 at 09:10:19AM -0800, Sean Christopherson wrote:

> @@ -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;

I suspect I might've been over eager here and we're now in trouble when
*_pmu_init() clears x86_pmu.guest_get_msrs (like for instance on AMD).

When that happens we need to restore __static_call_return0, otherwise
the following static_call_update() will patch in a NOP and RAX will be
garbage again.

So I've taken the liberty to update the patch as below.

---

Subject: x86/perf: Use RET0 as default for guest_get_msrs to handle "no PMU" case
From: Sean Christopherson <seanjc@google.com>
Date: Tue, 9 Mar 2021 09:10:19 -0800

From: Sean Christopherson <seanjc@google.com>

Initialize x86_pmu.guest_get_msrs to return 0/NULL to handle the "nop"
case.  Patching in perf_guest_get_msrs_nop() during setup does not work
if there is no PMU, as setup bails before updating the static calls,
leaving x86_pmu.guest_get_msrs NULL and thus a complete nop.  Ultimately,
this causes VMX abort on VM-Exit due to KVM putting random garbage from
the stack into the MSR load list.

Add a comment in KVM to note that nr_msrs is valid if and only if the
return value is non-NULL.

Fixes: abd562df94d1 ("x86/perf: Use static_call for x86_pmu.guest_get_msrs")
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Reported-by: syzbot+cce9ef2dd25246f815ee@syzkaller.appspotmail.com
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210309171019.1125243-1-seanjc@google.com
---

v2:
 - Use __static_call_return0 to return NULL instead of manually checking
   the hook at invocation.  [Peter]
 - Rebase to tip/sched/core, commit 4117cebf1a9f ("psi: Optimize task
   switch inside shared cgroups").

 arch/x86/events/core.c |   15 ++++++---------
 arch/x86/kvm/vmx/vmx.c |    2 +-
 2 files changed, 7 insertions(+), 10 deletions(-)

--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -81,7 +81,11 @@ DEFINE_STATIC_CALL_NULL(x86_pmu_swap_tas
 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_RET0(x86_pmu_guest_get_msrs, *x86_pmu.guest_get_msrs);
 
 u64 __read_mostly hw_cache_event_ids
 				[PERF_COUNT_HW_CACHE_MAX]
@@ -1944,13 +1948,6 @@ static void _x86_pmu_read(struct perf_ev
 	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;
@@ -2025,7 +2022,7 @@ static int __init init_hw_perf_events(vo
 		x86_pmu.read = _x86_pmu_read;
 
 	if (!x86_pmu.guest_get_msrs)
-		x86_pmu.guest_get_msrs = perf_guest_get_msrs_nop;
+		x86_pmu.guest_get_msrs = (void *)&__static_call_return0;
 
 	x86_pmu_static_call_update();
 
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6580,8 +6580,8 @@ static void atomic_switch_perf_msrs(stru
 	int i, nr_msrs;
 	struct perf_guest_switch_msr *msrs;
 
+	/* Note, nr_msrs may be garbage if perf_guest_get_msrs() returns NULL. */
 	msrs = perf_guest_get_msrs(&nr_msrs);
-
 	if (!msrs)
 		return;