linux-kernel - Re: [PATCH v1 5/5] KVM: Expose a version 1 architectural PMU to guests

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Date:	Sun, 05 Jun 2011 11:12:18 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
Subject: Re: [PATCH v1 5/5] KVM: Expose a version 1 architectural PMU to guests

On 06/03/2011 05:41 PM, Peter Zijlstra wrote:
> On Wed, 2011-05-11 at 11:55 -0400, Avi Kivity wrote:
> >  - counters that have PMI (interrupt) enabled stop counting after the
> >    interrupt is signalled.  This is because we need one-shot samples
> >    that keep counting, which perf doesn't support yet
>
> You'll have to reprogram the thing anyway, since not all hardware has
> the same counter width:
>
> [    0.046996] Performance Events: AMD PMU driver.
> [    0.048998] ... bit width:              48
>
> vs
>
> [    0.026998] Performance Events: PEBS fmt0+, Core2 events, Intel PMU driver.
> [    0.026998] ... bit width:              40
>
> simply letting the thing run will not behave in a consistent fashion.

I don't really follow.  How is the bit width related?

We adjust for bit width during PMC read.

Though I agree for accurate emulation we do need to reprogram, so we can 
set the next overflow event at 2^bit_width.  I doubt that anyone relies 
on this second overflow (even with 40 bits counting cycles, it takes far 
too long to overflow), still we have to emulate it.

> Or are you going to assume all software will properly read the cpuid
> leaf and not assume bit width?
>
> Also, I can't seem to locate where you fill that cpuid-leaf,
> kvm_pmu_cpuid_update() seems to read the entry, not write it.

We rely on host userspace to set up cpuid (and KVM_GET_SUPPORTED_CPUID 
to report to userspace what we support).

-- 
error compiling committee.c: too many arguments to function

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