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

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Date:   Tue, 14 Feb 2017 15:24:16 +0200
From:   Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:     Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Ingo Molnar <mingo@...hat.com>
Cc:     linux-kernel@...r.kernel.org, vince@...ter.net, eranian@...gle.com,
        Arnaldo Carvalho de Melo <acme@...radead.org>,
        Borislav Petkov <bp@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Subject: [PATCH 2/2] perf/x86/intel/pt: Fail event creation if VMX operation is on

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@...ux.intel.com>
Reported-by: Andi Kleen <ak@...ux.intel.com>
---
 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