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

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Date:	Fri, 20 Jul 2012 17:35:39 -0600
From:	David Ahern <dsahern@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Avi Kivity <avi@...hat.com>, Gleb Natapov <gleb@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: perf with precise attribute kills all KVM based VMs

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)
>                          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++;
>

Tested-by: David Ahern <dsahern@...il.com>

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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