linux-kernel - Re: perf with precise attribute kills all KVM based VMs

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Date:	Mon, 09 Jul 2012 16:47:48 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Avi Kivity <avi@...hat.com>
Cc:	Gleb Natapov <gleb@...hat.com>, David Ahern <dsahern@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: perf with precise attribute kills all KVM based VMs

On Mon, 2012-07-09 at 17:39 +0300, Avi Kivity wrote:
> > Disabling PEBS events for guests isn't pretty though.. 
> 
> We already have atomic MSR switching at guest entry/exit time.  So it's
> not pretty in terms of not getting full profiling, but the code won't be
> too hard.  Basically we just have to exclude_guest any pebs event. 

OK, so ideally we'd do something like the below, except of course that
that isn't backwards compatible and will break the world :/

bugger that

---
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -336,6 +338,9 @@ int x86_setup_perfctr(struct perf_event *event)
                /* BTS is currently only allowed for user-mode. */
                if (!attr->exclude_kernel)
                        return -EOPNOTSUPP;
+
+               if (!attr->exclude_guest)
+                       return -EOPNOTSUPP;
        }
 
        hwc->config |= config;
@@ -378,6 +383,9 @@ int x86_pmu_hw_config(struct perf_event *event)
        if (event->attr.precise_ip) {
                int precise = 0;
 
+               if (!event->attr.exclude_guest)
+                       return -EOPNOTSUPP;
+
                /* Support for constant skid */
                if (x86_pmu.pebs_active && !x86_pmu.pebs_broken) {
                        precise++;
--
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