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

From: Scott Wood
Subject: Re: [Qemu-ppc] [kvm-devel] [PATCH v2] kvm-ppc: halt secondary cpus when guest reset
Date: Tue, 10 Jan 2012 16:43:14 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:6.0.2) Gecko/20110906 Thunderbird/6.0.2

On 01/10/2012 11:52 AM, Jan Kiszka wrote:
> On 2012-01-10 18:43, Scott Wood wrote:
>> 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.
> Where is the state stored that declares a VCPU to wait for that event?
> Where is it set, where removed?
> What about implementing MP_STATE on PPC, at least those states that make
> sense? Don't you need that anyway for normal HALT<->RUNNABLE transitions?

On ppc, normal halt/runnable transitions are handled entirely in the
kernel, even without irqchip.

So, the idea is that on secondary VCPU creation, QEMU sets MP_STATE to
KVM_MP_STATE_UNITIALIZED, and KVM will hold the thread idle until the
MMIO is done and QEMU sets MP_STATE to KVM_MP_STATE_RUNNABLE?  It seems
excessive compared to QEMU being able to figure out for itself when it
doesn't want to run a VCPU thread, when the decision is based entirely
on things that are modeled in QEMU (which it will still need to do in
the non-KVM case).


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