Re: [Qemu-devel] [PATCH] Fix irq route entries exceed KVM_MAX_IRQ_ROUTES

[Paolo Bonzini]

On 31/12/2014 04:45, kevinnma(马文霜) wrote:
> Last month, we experienced several guests crash(6cores-8cores),qemu logs
> display the following messages:
> 
> qemu-system-x86_64: /build/qemu-2.1.2/kvm-all.c:976:
> kvm_irqchip_commit_routes: Assertion `ret == 0' failed.
> 
> After analysis and verification, we can confirm it's irq-balance
> daemon(in guest) leads to the assertion failure.So start a 8 core guest
> with two disks, execute the following scripts will reproduce the BUG quickly:
> 
> vda_irq_num=25
> vdb_irq_num=27
> while [ 1 ]
> do
>     for irq in {1,2,4,8,10,20,40,80}
>         do
>             echo $irq > /proc/irq/$vda_irq_num/smp_affinity
>             echo $irq > /proc/irq/$vdb_irq_num/smp_affinity
>             dd if=/dev/vda of=/dev/zero bs=4K count=100 iflag=direct
>             dd if=/dev/vdb of=/dev/zero bs=4K count=100 iflag=direct
>         done
> done
> 
> QEMU setup static irq route entries in kvm_pc_setup_irq_routing(),PIC and
> IOAPIC share the first 15 GSI numbers,take up 23 GSI numbers,but take up 38
> irq route entries.When change irq smp_affinity in guest,a dynamic route
> entry may be setup,the current logic is:if allocate GSI number succeeds,
> a new route entry can be added.The available dynamic GSI numbers is
> 1021(KVM_MAX_IRQ_ROUTES-23),but available irq route entries is only
> 986(KVM_MAX_IRQ_ROUTES-38),GSI numbers greater than route entries.
> irq-balance's behavior will eventually leads to total irq route entries
> exceed KVM_MAX_IRQ_ROUTES,ioctl(KVM_SET_GSI_ROUTING) fail and
> kvm_irqchip_commit_routes() trigger assertion failure.

I have two questions:

1) why isn't the existing check in kvm_irqchip_get_virq enough to fix
the bug?

    if (!s->direct_msi && retry) {
        retry = false;
        kvm_flush_dynamic_msi_routes(s);
        goto again;
    }

2) If you introduce this extra call to kvm_flush_dynamic_msi_routes,
does the existing check become obsolete?

Thanks,

Paolo