[PATCH] ARM: KVM: iterate over all CPUs for CPU compatibility check

[Christoffer Dall]

On Wed, Apr 17, 2013 at 3:35 AM, Marc Zyngier <marc.zyngier at arm.com> wrote:
> On 17/04/13 11:19, Russell King - ARM Linux wrote:
>> On Fri, Apr 12, 2013 at 02:49:43PM +0100, Marc Zyngier wrote:
>>> On 12/04/13 14:40, Peter Maydell wrote:
>>>> On 12 April 2013 14:24, Marc Zyngier <marc.zyngier at arm.com> wrote:
>>>>> Nak. The fact that one of the CPUs seem to hang is a sure sign that
>>>>> something is severely broken, and you definitely want to fix that issue,
>>>>> instead of blindly ignoring it.
>>>>>
>>>>> Additionally, it seems you're just papering over the issue. You should
>>>>> be able to exclude the A7 processors, but not completely deny KVM from
>>>>> running on the hardware.
>>>>
>>>> Well that might be nice, as would fully supporting big.LITTLE
>>>> systems. But until somebody actually does that work it seems
>>>> like a better idea to fail gracefully rather than having a 50%
>>>> chance of failing gracefully and a 50% chance of going weird.
>>>
>>> Nothing prevents the kernel (or even the user) from forcing the affinity
>>> of the CPU threads to the A15s. I'm not saying we should ignore the
>>> problem either. Just that the proposed approach is wrong.
>>
>> But nothing guarantees that you get that affinity.  If you offline all
>> A15 CPUs, then you will find those threads running on whatever is left.
>> Affinity is just a hint, nothing more.
>
> I completely agree with you. But if we're left with only CPUs we can't
> run on, we're screwed and must abort.
>
> It's the same story as the RealView PB-X, where only one of the two A9
> has NEON. If the NEON-capable core is down, any process using NEON is
> virtually dead.
>
> Should that be a reason to completely disable the HW (in this case the 3
> A7s)? I'm not sure...
>
But we're not talking about disabling the A7's, we're talking about
disabling KVM/ARM, a quite new feature, on a system where it's not
well-tested and may cause boot problems or other issues that we
haven't investigated in depth yet.