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

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, May 04, 2009 at 12:21:28PM +0300, Avi Kivity wrote:
> 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.

Since guest drivers are going to do round-robin most of the time, I
think the right thing to do is to make the API simple, along the lines
proposed by Rusty, and make the guest/host ABI rich enough to support
arbitrary mapping, along the lines proposed by you. We can always change
the API, ABI is harder.

-- 
MST