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From: Peter Maydell <peter.maydell@linaro.org>
Date: Mon, 1 Aug 2016 14:32:52 +0100
Message-ID: <CAFEAcA97p9GFrp+tX3pwqeVaSxjxVwuPvCAgZt5aYd_Yk+mGnA@mail.gmail.com>
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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: Andrea Bolognani <abologna@redhat.com>
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 1 August 2016 at 14:26, Andrea Bolognani <abologna@redhat.com> wrote:
> On Mon, 2016-08-01 at 15:08 +0200, Andrew Jones wrote:
>> > I'm not sure a warning is enough: if I start a guest and
>> > explicitly ask for a PMU, I expect it to be there, or for
>> > the guest not to start at all. How does x86 behave in this
>> > regard?
>>
>> Peter had a good suggestion for this. We need to wrap the property
>> addition in an arm_feature check like the has_el3 property. That will
>> remove it from all cpu types that don't support it.
>
> Wouldn't that mean that you'd be unable to use
>
>   -cpu foo,pmu=off
>
> if CPU model 'foo' doesn't support a PMU? I'd expect that
> to work.

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.

thanks
-- PMM