Re: [PATCH 08/11] perf tool: precise mode requires exclude_guest

[Robert Richter]

David,

On 06.09.12 00:07:52, Peter Zijlstra wrote:
> On Wed, 2012-09-05 at 09:43 -0600, David Ahern wrote:
> > We need to require exclude_guest when using precise attribute with perf 
> > else all running VMs on Intel-based servers will crash. I do not have an 
> > AMD based server to even attempt the preferred solution.

do you have a perf tool patch ready that enables exclude_guest per
default for precise events? I have ibs kernel code that returns EINVAL
in this case.  Current perf tool should then fall back by sending
another syscall with the exclude_guest modifier disabled. This should
work for ibs. Testing your patch wouln't be an effort for me.

To prevent Intel-based servers from crashing, pebs should be only
enabled if the exclude_guest modifier is enabled. I guess there is
already kernel code to prevent this, but didn't look at the sources.

To prevent probing the kernel with syscalls to detect pmu features, I
suggested some time ago to introduce

 /sys/bus/event_source/devices/*/flags

or so similar to /proc/cpuinfo:flags. Any opinions on that?

There is also the option to emulate exclude_guest for ibs in
software. So far I didn't have the time to look at this. We could
still add this in the future. Since we need to fix current mainline
anyway where such a patch wouldn't go in for v3.6, I will send my
current ibs kernel fixes I mentioned above next days. It would be
great to test this also with your patch.

Thanks,

-Robert

-- 
Advanced Micro Devices, Inc.
Operating System Research Center

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