Re: [PATCH RFC 0/8] virtio: add guest MSI-X support

[Avi Kivity <avi@redhat.com>]

Michael S. Tsirkin wrote:
>>> So what I see is transports providing something like:
>>>
>>> struct virtio_interrupt_mapping {
>>> 	int virtqueue;
>>> 	int interrupt;
>>> };
>>>
>>> map_vqs_to_interrupt(dev, struct virtio_interrupt_mapping *, int nvirtqueues);
>>> unmap_vqs(dev);
>>>   
>>>       
>> Isn't that the same thing?  Please explain the flow.
>>     
>
> So to map vq 0 to vector 0, vq 1 to vector 1 and vq 2 to vector 2 the driver would do:
>
> struct virtio_interrupt_mapping mapping[3] = { {0, 0}, {1, 1}, {2, 2} };
> vec = map_vqs_to_interrupt(dev, mapping, 3);
> if (vec) {
>   error handling
> }
>
> and then find_vq as usual.
>   

Yes, that works.

Given that pci_enable_msix() can fail, we can put the retry loop in 
virtio-pci, and instead of a static mapping, supply a dynamic mapping:

    static void get_vq_interrupt(..., int nr_interrupts, int vq)
    {
        /* reserve interrupt 0 to config changes; round-robin vqs to 
interrupts */
        return 1 + (vq % (nr_interrupts - 1));
    }

    driver_init()
    {
        map_vqs_to_interrupt(dev, get_vq_interrupt);
    }

map_vqs_to_interrupts() would call get_vq_interrupt() for each vq, 
assuming the maximum nr_interrupts, and retry with smaller nr_interrupts 
on failure.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.