On Wed, 15 Jul 2020 17:40:33 +0100 Alex Bennée <alex.ben...@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. --------------------------------------------------------------------- To unsubscribe, e-mail: virtio-dev-unsubscr...@lists.oasis-open.org For additional commands, e-mail: virtio-dev-h...@lists.oasis-open.org
