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From: Andrea Bolognani <abologna@redhat.com>
Date: Mon, 01 Aug 2016 16:55:01 +0200
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Subject: Re: [Qemu-devel] [PATCH RFC 1/1] arm64: add an option to turn
 on/off vpmu support
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To: Peter Maydell <peter.maydell@linaro.org>
Cc: Andrew Jones <drjones@redhat.com>, Wei Huang <wei@redhat.com>, qemu-arm <qemu-arm@nongnu.org>, QEMU Developers <qemu-devel@nongnu.org>, Shannon Zhao <shannon.zhao@linaro.org>

On Mon, 2016-08-01 at 14:32 +0100, Peter Maydell wrote:
> > Wouldn't that mean that you'd be unable to use
> >=C2=A0
> >=C2=A0=C2=A0=C2=A0-cpu foo,pmu=3Doff
> >=C2=A0
> > if CPU model 'foo' doesn't support a PMU? I'd expect that
> > to work.
>=C2=A0
> The current precedent (has_el3) doesn't work like that: if
> foo isn't a CPU which can support EL3 then the property doesn't
> exist, and it's an error to try to set it.

Doesn't look like the pmu option works like that on x86,
though, unless I'm missing something.

I have a guest running with

=C2=A0 -cpu pentium,pmu=3Don

and I can't see hardware perf events from inside the guest,
eg. dmesg reports

=C2=A0 Performance Events: no PMU driver, software events only.

and perf tells me

=C2=A0 <not supported> instructions
=C2=A0 <not supported> branches
=C2=A0 ...

I'm not sure whether that's because the PMU is being
emulated but the kernel doesn't have a driver for it, or
whether it's not being emulated at all. Any way to find
out?

--=C2=A0
Andrea Bolognani / Red Hat / Virtualization