Re: [KVM PATCH v4 3/3] kvm: add iosignalfd support

[Mark McLoughlin <markmc@redhat.com>]

On Wed, 2009-05-27 at 16:45 -0400, Gregory Haskins wrote:
> Mark McLoughlin wrote:
> > The virtio ABI is fixed, so we couldn't e.g. have the guest use a cookie
> > to identify a queue - it's just going to continue using a per-device
> > queue number. 
> 
> Actually, I was originally thinking this would be exposed as a virtio
> FEATURE bit anyway, so there were no backwards-compat constraints.  That
> said, we can possibly make it work in a backwards compat way, too. 
> IIRC, today virtio does a PIO cycle to a specific register with the
> queue-id when it wants to signal guest->host, right?  What is the width
> of the write?

It's a 16-bit write.

/* A 16-bit r/w queue notifier */
#define VIRTIO_PCI_QUEUE_NOTIFY         16

> > So, if the cookie was also the trigger, we'd need an
> > eventfd per device.
> >   
> 
> I'm having trouble parsing this one.  The cookie namespace is controlled
> by the userspace component that owns the corresponding IO address, so
> there's no reason you can't make "queue-id = 0" use cookie = 0, or
> whatever.  That said, I still think a separation of the cookie and
> trigger as suggested above is a good idea, so its probably moot to
> discuss this point further.

Ah, my mistake - I thought the cookie was returned to userspace when the
eventfd was signalled, but no ... userspace only gets an event counter
value and the cookie is used during de-assignment to distinguish between
iosignalfds.

Okay, so suppose you do assign multiple times at a given address -
you're presumably going to use a different eventfd for each assignment?
If so, can't we match using both the address and eventfd at
de-assignment and drop the cookie from the interface altogether?

i.e. to replace the virtio queue notify with this, we'd:

  1) create an eventfd per queue

  2) assign each of those eventfds to the QUEUE_NOTIFY address

  3) have only one of the eventfds be triggered, depending on what 
     value is written by the guest

  4) de-assign using the address/eventfd pair to distinguish between 
     assignments

Cheers,
Mark.