Re: perf with precise attribute kills all KVM based VMs

From: David Ahern
Date: Fri Jul 20 2012 - 19:35:37 EST

On 7/9/12 8:47 AM, Peter Zijlstra wrote:
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)
+ 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) {

Tested-by: David Ahern <dsahern@xxxxxxxxx>

Gleb's patch is required as well. It takes both to fix the problem. And my userspace patch keeps compatibility for users who currently use -e cycles:pp (they won't be forced to add 'H'). (lkml is slow to show it so I can't pull a link for reference, but it's in your email ahead of this one).

Be nice to get this set into stable releases as well.
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