2016-12-20 19:59+0800, Wanpeng Li:
> 2016-11-24 20:42 GMT+08:00 Radim Krčmář <rkrcmar@xxxxxxxxxx>:
>> 2016-11-23 22:58+0100, Paolo Bonzini:
>>> On 23/11/2016 21:25, Radim Krčmář wrote:
>>>> diff --git a/arch/x86/kvm/irq_comm.c b/arch/x86/kvm/irq_comm.c
>>>> index 25810b144b58..ddd63b8b176e 100644
>>>> --- a/arch/x86/kvm/irq_comm.c
>>>> +++ b/arch/x86/kvm/irq_comm.c
>>>> @@ -41,6 +41,15 @@ static int kvm_set_pic_irq(struct kvm_kernel_irq_routing_entry *e,
>>>> bool line_status)
>>>> {
>>>> struct kvm_pic *pic = pic_irqchip(kvm);
>>>> +
>>>> + /*
>>>> + * XXX: rejecting pic routes when pic isn't in use would be better,
>>>> + * but the default routing table is installed while kvm->arch.vpic is
>>>> + * NULL and KVM_CREATE_IRQCHIP can race with KVM_SET_GSI_ROUTING.
>>>> + */
>>>> + if (!pic)
>>>> + return -1;
>>>> +
>>>> return kvm_pic_set_irq(pic, e->irqchip.pin, irq_source_id, level);
>>>> }
>>>>
>>>
>>> Can you explain the race with the default routing table better? It
>>> seems to me that it can only make the routing table go from invalid to
>>> valid (there is no KVM_DESTROY_IRQCHIP) so it's benign.
>>
>> Oops, I wrote the race with wrong IOCTL -- it should be KVM_IRQ_LINE.
>>
>> 1) set KVM_CAP_SPLIT_IRQCHIP (unlocks KVM_IRQ_LINE)
>> a) call KVM_CREATE_IRQCHIP (creates routes while !kvm->arch.vpic)
>> b) concurrently call KVM_IRQ_LINE for PIO routes (dereferences NULL)
>
> If we should not go through irqfd if irqchip is split?
I also remember hearing about that -- do you remember where it was?
The documentation does not say that and irqfd is mostly optimization for
KVM_IRQ_LINE ... QEMU uses KVM_IRQ_LINE_STATUS with split irqchip, so we
can't easily say the opposite now.
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