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

[Avi Kivity <avi@redhat.com>]

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.

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

-- 
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