Re: [Qemu-devel] [PATCH v2 2/3] virtio-mmio: introduce set_guest_notifiers

[Cornelia Huck]

On Fri, 08 May 2015 09:45:00 +0300
Pavel Fedin <address@hidden> wrote:

>  Hello!
> 
> > Hm, weren't there some patches for irqfd on arm?
> 
>  Yes, there were. However, they had a design problem by breaking backwards 
> compatibility
> with unmodified virtio. Their idea was to set up one more shared memory area 
> between
> virtio and vhost-net and use it to pass ISR value, which helps to 
> distinguish, which event
> took place (queue update or config change). So, this idea was rejected.
>  http://lists.gnu.org/archive/html/qemu-devel/2014-10/msg03056.html
> 
>  I thought about it, and technically, i think, this can be done in better 
> way. Actually,
> as far as i understood, all we need is mechanism for distinguishing between 
> these two
> events. On PCI we do this by using multiple IRQs via MSI-X, and one IRQ 
> signals exactly
> one type of event. MSI-X code also has "two IRQs" mode as a failsafe, where 
> one IRQ
> signals config change and another IRQ signals queues update (and all queues 
> are polled in
> turn). I think a similar thing could be done for virtio-mmio. It could 
> allocate two IRQs
> instead of one and describe both of them in the device tree. Guest side, upon 
> seeing that,
> could make use of those two IRQs and acknowledge to the host side that "yes, 
> i am new
> version and use new mode".
>  But, sorry, i will unlikely implement this, because we already have PCI with 
> MSI-X (i
> hope this is going to be published soon), so my project can use PCI 
> emulation. So
> implementing irqfds for virtio-mmio is a bit out of my scope.

Thanks for the explanation.

Yes, I think it makes sense to just pick the low-hanging fruit for
virtio-mmio and wait for pci.