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

[Avi Kivity <avi@redhat.com>]

Michael S. Tsirkin wrote:
> 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.
>   

We can provide the round-robin mapper as a helper, so driver code 
doesn't need to implement any callback if they're satisfied with the 
default.

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