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

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

On Mon, Apr 27, 2009 at 06:06:48PM +0300, Avi Kivity wrote:
> Christian Borntraeger wrote:
>> Am Monday 27 April 2009 14:31:36 schrieb Michael S. Tsirkin:
>>   
>>> Add optional MSI-X support: use a vector per virtqueue with
>>> fallback to a common vector and finally to regular interrupt.
>>> Teach all drivers to use it.
>>>
>>> I added 2 new virtio operations: request_vqs/free_vqs because MSI
>>> needs to know the total number of vectors upfront.
>>>
>>> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
>>>     
>>
>> I dont know, if that is feasible for MSI, but the transport(virtio_pci) should
>> already know the number of virtqueues, which should match the number of
>> vectors, no?
>>
>> In fact, the transport has to have a way of getting the number of virtqeues
>> because find_vq returns ENOENT on invalid index numbers.
>>   
>
> One thing I thought was to decouple the virtqueue count from the MSI  
> entry count, and let the guest choose how many MSI entries it wants to  
> use.  This is useful if it wants several queues to share an interrupt,  
> perhaps it wants both rx and tx rings on one cpu to share a single 
> vector.
>
> So the device could expose a large, constant number of MSI entries, and  
> in addition expose a table mapping ring number to msi entry number.  The  
> guest could point all rings to one entry, or have each ring use a  
> private entry, or anything in between.

Sounds good ... and it might not be a lot of code I think.

> That saves us the new API (at the expense of a lot more code, but with  
> added flexibility).

So we'll probably need to rename request_vqs to request_vectors,
but we probably still need the driver to pass the number of
vectors it wants to the transport. Right?

-- 
MST