[PATCH 2/2] perf/x86/intel/pt: Fail event creation if VMX operation is on

[Alexander Shishkin]

On systems where PT does not coexist with VMX, users get confused when
PT turns up with no data because they forgot they're running a kvm
session at the same time.

This patch adds a preemptive check for any active VMX operations that
will fail event creation. This does not provide any guarantees or
protection against racing with a kvm starting in parallel, but is
intended to serve as a hint for the user. If VMXON happens after an
event had been created, the event will still produce an empty trace.

Signed-off-by: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
Reported-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
---
 arch/x86/events/intel/pt.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/arch/x86/events/intel/pt.c b/arch/x86/events/intel/pt.c
index 9372fa4549..b1490a879c 100644
--- a/arch/x86/events/intel/pt.c
+++ b/arch/x86/events/intel/pt.c
@@ -1444,6 +1444,20 @@ static void pt_event_destroy(struct perf_event *event)
 
 static int pt_event_init(struct perf_event *event)
 {
+	int cpu, vmx_on = 0;
+
+	get_online_cpus();
+	for_each_online_cpu(cpu) {
+		struct pt *pt = per_cpu_ptr(&pt_ctx, cpu);
+
+		if (READ_ONCE(pt->vmx_on))
+			vmx_on++;
+	}
+	put_online_cpus();
+
+	if (vmx_on)
+		return -EBUSY;
+
 	if (event->attr.type != pt_pmu.pmu.type)
 		return -ENOENT;
 
-- 
2.11.0