[Qemu-devel] Re: [PATCH 2/2] virtio: enable msi-x for console+balloon

[Gerd Hoffmann <kraxel@redhat.com>]

On 12/07/09 11:59, Michael S. Tsirkin wrote:
>> The motivation was to move them away from the ioapic, to reduce irq
>> sharing of *other* devices which are connected to the ioapic too (i.e.
>> usb, e1000, lsi, ...)
>
> Let's convert these to MSI instead?
> This will likely pay off long term,
> e.g. with nested virtualization.

Works only of the real hardware we emulate can do MSI-X too, otherwise 
guests simply wouldn't use it.  I think the only case where this could 
work out is e1000, newer revisions can do MSI-X.  Maybe also the 
upcoming megasas emulation Hannes is working on.

>> I'm aware that these are not performance-critical.  I've even tried to
>> use 'vectors=1' because of that.  I expected that would make them use
>> MSI-X, but a single IRQ line only.  Didn't work though.  Intentional?
>
> So it's even worse, we are using up 2 vectors per device? Ugh ...

Yes.  Three for balloon, but that can easily changed to two.

> no way to distinguish between vq interrupt
> and config change. This last thing is important
> because it allows fastpath injection of MSI
> interrupts directly from kernel without
> notifying qemu to update IRQ field.

Ah, *that* is the reason for the separate config interrupt.  Does the 
in-kernel injection matter for balloon+console?  I'd expect only 
virtio-net needs that when it is configured with vhost?

cheers
   Gerd