Re: [PATCH] KVM: x86: Kick VCPU outside PIC lock again

Avi Kivity
2010-02-24T10:41:42Z

On 02/24/2010 12:28 PM, Jan Kiszka wrote:
> Jan Kiszka wrote:
>
>> Avi Kivity wrote:
>>
>>> On 02/24/2010 12:13 PM, Jan Kiszka wrote:
>>>
>>>>
>>>>
>>>>> I see. Won't we hit the same issue when we call pic functions from
>>>>> atomic context during the guest entry sequence?
>>>>>
>>>>>
>>>>>
>>>> If there are such call paths, for sure. What concrete path(s) do you
>>>> have in mind?
>>>>
>>>>
>>>>
>>> vcpu_enter_guest() -> inject_pending_event() ->
>>> kvm_cpu_{has,get}_interrupt() -> various pic functions if you're unlucky.
>>>
>> But do they kick anyone or just check/pull information? Never saw any
>> warnings during my tests last year (granted: with older -rt and kvm
>> versions).
>>
> Mmh, they could if there is> 1 IRQ pending. Guess this just never
> triggered in real life due to typical APIC use and low IRQ load during
> PIC times in my tests.
>

We could just ignore the wakeup in this path. It's called in vcpu
context, so obviously the vcpu is awake and kicking it will only hurt
your feet.

Longer term, we should clear up this mess. One possible path is to move
the pic/lapic/injection stuff out of the the critical section, and use
vcpu->requests to close the race between querying the pic/lapic and
entering the guest.

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