Re: [virtio-dev] On doorbells (queue notifications)

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 15 Jul 2020 17:40:33 +0100
Alex BennÃe <alex.bennee@linaro.org> wrote:

> I was also wondering about the efficiency of doorbells/notifications the
> other way. AFAIUI for both PCI and MMIO only a single write is required
> to the notify flag which causes a trap to the hypervisor and the rest of
> the processing. The hypervisor doesn't have the cost multiple exits to
> read the guest state although it obviously wants to be as efficient as
> possible passing the data back up to what ever is handling the backend
> of the device so it doesn't need to do multiple context switches.
> 
> Has there been any investigation into other mechanisms for notifying the
> hypervisor of an event - for example using a HYP call or similar
> mechanism?

For ccw devices, we do a 'diagnose' call (which is basically a
hypercall). It has some parameters (including the queue the guest is
notifying for), so the host already has that information when the exit
happens.

> My gut tells me this probably doesn't make any difference as a trap to
> the hypervisor is likely to cost the same either way because you still
> need to save the guest context before actioning something but it would
> be interesting to know if anyone has looked at it. Perhaps there is a
> benefit in partitioned systems where core running the guest can return
> straight away after initiating what it needs to internally in the
> hypervisor to pass the notification to something that can deal with it?

I guess that depends mostly upon whether there is further interaction
needed, or if the guest can give the host all needed info in one go.