linux-kernel - Re: [PATCH] x86/perf: Fix virtualization sanity check

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Date:	Tue, 09 Oct 2012 17:41:44 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andre Przywara <andre.przywara@....com>
Cc:	mingo@...e.hu, konrad.wilk@...cle.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/perf: Fix virtualization sanity check

On Tue, 2012-10-09 at 17:38 +0200, Andre Przywara wrote:
> First you need an AMD family 10h/12h CPU. These do not reset the
> PERF_CTR registers on a reboot.
> Now you boot bare metal Linux, which goes successfully through this
> check, but leaves the magic value of 0xabcd in the register. You
> don't use the performance counters, but do a reboot (warm reset).
> Then you choose to boot Xen. The check will be triggered with a
> recent Linux kernel as Dom0 again, trying to write 0xabcd into the
> MSR. Xen silently drops the write (expected), but the subsequent read
> will return the value in the register, which just happens to be the
> expected magic value. Thus the test misleadingly succeeds, leaving
> the kernel in the belief that the PMU is available 

Wow.. ! that's uhm.. shees!

Bit weird of Xen to trap writes but not reads of MSRs though.

The patchs looks fine though, thanks!
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