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Date: Tue, 09 Jun 2015 15:00:23 +0100
From: Marc Zyngier <marc.zyngier@arm.com>
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Subject: Re: [Qemu-devel] should KVM or userspace be the one which decides
 what MIPIDR/affinity values to assign to vcpus?
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To: Peter Maydell <peter.maydell@linaro.org>
Cc: Shlomo Pongratz <shlomo.pongratz@huawei.com>, Shlomo Pongratz <shlomopongratz@gmail.com>, Pavel Fedin <p.fedin@samsung.com>, QEMU Developers <qemu-devel@nongnu.org>, Shannon Zhao <shannon.zhao@linaro.org>, Ashok Kumar <ashoks@broadcom.com>, Igor Mammedov <imammedo@redhat.com>, "kvmarm@lists.cs.columbia.edu" <kvmarm@lists.cs.columbia.edu>

On 09/06/15 14:16, Peter Maydell wrote:
> On 9 June 2015 at 13:27, Marc Zyngier <marc.zyngier@arm.com> wrote:
>> On 09/06/15 12:24, Peter Maydell wrote:
>>> So either:
>>>  * QEMU needs to tell the kernel the MPIDR for each vCPU
>>>  * QEMU needs to ask the kernel the MPIDR for each vCPU
>>>
>>> Which is better? The latter is simpler and will work with
>>> existing kernels. The former would let us (for instance)
>>> use KVM when we're modelling (real world) boards which have
>>> particular cluster configurations (which might not match the
>>> kernel's current simplistic "always 16 CPUs at Aff0" setup).
>>
>> Given that it is userspace that provides the description of the
>> platform, it feels natural to let userspace set MPIDR accordingly.
>>
>> This will require some interesting rework in the kernel, but this
>> doesn't feel too bad.
> 
> On the other hand QEMU needs to keep working with current kernels,
> so we need the "ask the kernel" code at least for the moment.
> Probably needs a capability for "kernel allows userspace to
> set MPIDR" so we can adapt.

Yeah, what I had in mind was something along the lines of:
- kernel computes its "default MPDIR"
- kernel exposes a new capability "KVM_ARM_ALLOW_MPIDR_OVERRIDE" (or
something along those lines)
- userspace does the right thing.

	M.
-- 
Jazz is not dead. It just smells funny...