Re: [Qemu-ppc] [kvm-devel] [PATCH v2] kvm-ppc: halt secondary cpus when guest reset

[Scott Wood]

On 01/10/2012 03:38 AM, Jan Kiszka wrote:
> On 2012-01-10 00:17, Scott Wood wrote:
>> On 01/09/2012 04:39 PM, Alexander Graf wrote:
>>>
>>> On 09.01.2012, at 22:23, Scott Wood wrote:
>>>> Alex, is there a better way to deal with the IRQ chip issue?
>>>
>>> To be honest, I'm not sure what the issue really is.
>>
>> If irqchip is enabled, env->halted won't result in a CPU being
>> considered idle -- since QEMU won't see the interrupt that wakes the
>> vcpu, and the idling is handled in the kernel.  In this case we're
>> waiting for MMIO rather than an interrupt, and it's the kernel that
>> doesn't know what's going on.
>>
>> It seems wrong to use env->stopped, though, as a spin-table release
>> should not override a user's explicit request to stop a CPU.  It might
>> be OK (though a bit ugly) if the only usage of env->stopped is through
>> pause_all_vcpus(), and the boot thread is the first one to be kicked
>> (though in theory the boot cpu could wake another cpu, and that could
>> wake a cpu that comes before it, causing a race with pause_all_vcpus()).
>>
>> If it is OK to use env->stopped, is there any reason not to always use
>> it (versus just with irqchip)?
> 
> Why don't you wait in the kernel with in-kernel irqchip under all
> condition (except pausing VCPUs, of course) on PPC? Just like x86 does.

We do for normal idling.  This is a bit different, in that we're not
waiting for an interrupt, but for an MMIO that releases the cpu at
boot-time.

-Scott