Re: [Qemu-devel] [PATCH] virtio: Implement userspace forwarding for host notifiers

[Cornelia Huck]

On Thu, 19 Nov 2015 15:17:57 +0300
Pavel Fedin <address@hidden> wrote:

>  Hello!
> 
> > > Paolo, do you see anything wrong with making
> > > memory_region_add_eventfd work (slowly) without kvm ioeventfd support?
> > >
> > 
> > Sure, it's even been on the todo list for a while.  Stefan Hajnoczi had
> > a patch, the only ugly thing was that it slowed down a little all
> > non-ioeventfd accesses, but I guess that's okay because it's probably
> > not measurable.
> > 
> > http://lists.nongnu.org/archive/html/qemu-devel/2015-07/msg04710.html

If I read this correctly, memory regions already keep track of
ioeventfds and this patch can simply trigger them manually if that had
not already been done?

> 
>  Ok, good. So, this has already been tested. Ok, i think i can further extend 
> this patch by:
> 1. Add a flag to disable memory_region_dispatch_write_ioeventfds(), to be set 
> by KVM code for example. So that with KVM we have even
> smaller performance degradation.
> 2. Issue a one-shot warning, like in my current patch, so that the user knows 
> that his/her kernel prevents from getting the best
> performance.
>  Will it be OK then?
> 
>  Cc'ed also Cornelia because he's taking part in CCW discussion. And the same 
> kind of generic event forwarding could be implemented
> for CCW then.

Only that we don't have such a nice tracking structure that memory
regions already have.

The intercept handler for diagnose 500 already ends up at the correct
device (its id is one of the parameters), so I'd need to check for an
existing notifier on the virtqueue and trigger that instead of calling
virtio_queue_notify directly?