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Date: Thu, 7 Jan 2016 16:37:22 +0000
From: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
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Subject: Re: [Qemu-devel] arm64 qemu tests failing in linux-next since
 'arm64: kernel: enforce pmuserenr_el0 initialization and restore'
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To: Peter Maydell <peter.maydell@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>, Will Deacon <will.deacon@arm.com>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, QEMU Developers <qemu-devel@nongnu.org>, Guenter Roeck <linux@roeck-us.net>, "linux-arm-kernel@lists.infradead.org" <linux-arm-kernel@lists.infradead.org>

On Thu, Jan 07, 2016 at 03:58:15PM +0000, Peter Maydell wrote:
> On 7 January 2016 at 15:53, Lorenzo Pieralisi <lorenzo.pieralisi@arm.com> wrote:
> > On Thu, Jan 07, 2016 at 01:25:35PM +0000, Peter Maydell wrote:
> >> We had previously been relying on the kernel not attempting to
> >> touch the PMU if the ID_AA64DFR0_EL1 PMUVer bits read 0000
> >> ("Performance Monitors extension System registers not implemented").
> >
> > Ok, thanks for looking into this. I wonder why reading pmcr_el0 does
> > not suffer from the same problem though.
> 
> Just a pragmatic thing on QEMU's end, I expect -- the kernel already
> touched PMCR_EL0 and we wanted to be able to boot it, so we have an
> implementation of it.

If that's the case, that was the wrong approach IMHO. QEMU has to comply
with the Aarch64 architecture which means that either the CPU it models
has a Performance Monitors extension or it does not. If reading pmcr_el0
does not fault I could tell you this is a QEMU regression because currently
it _does_ model pmcr_el0 while (hopefully) ID_AA64DFR0_EL1 PMUVer reports
it should not.

I will add code that guards both register accesses to fix both bugs at
once.

Thanks,
Lorenzo