On Wed, Jun 28, 2023 at 4:15 PM Stefan Hajnoczi <stefa...@gmail.com> wrote: > > On Wed, 28 Jun 2023 at 09:59, Jason Wang <jasow...@redhat.com> wrote: > > > > On Wed, Jun 28, 2023 at 3:46 PM Stefan Hajnoczi <stefa...@gmail.com> wrote: > > > > > > On Wed, 28 Jun 2023 at 05:28, Jason Wang <jasow...@redhat.com> wrote: > > > > > > > > On Wed, Jun 28, 2023 at 6:45 AM Ilya Maximets <i.maxim...@ovn.org> > > > > wrote: > > > > > > > > > > On 6/27/23 04:54, Jason Wang wrote: > > > > > > On Mon, Jun 26, 2023 at 9:17 PM Ilya Maximets <i.maxim...@ovn.org> > > > > > > wrote: > > > > > >> > > > > > >> On 6/26/23 08:32, Jason Wang wrote: > > > > > >>> On Sun, Jun 25, 2023 at 3:06 PM Jason Wang <jasow...@redhat.com> > > > > > >>> wrote: > > > > > >>>> > > > > > >>>> On Fri, Jun 23, 2023 at 5:58 AM Ilya Maximets > > > > > >>>> <i.maxim...@ovn.org> wrote: > > > > > >> It is noticeably more performant than a tap with vhost=on in terms > > > > > >> of PPS. > > > > > >> So, that might be one case. Taking into account that just rcu > > > > > >> lock and > > > > > >> unlock in virtio-net code takes more time than a packet copy, some > > > > > >> batching > > > > > >> on QEMU side should improve performance significantly. And it > > > > > >> shouldn't be > > > > > >> too hard to implement. > > > > > >> > > > > > >> Performance over virtual interfaces may potentially be improved by > > > > > >> creating > > > > > >> a kernel thread for async Tx. Similarly to what io_uring allows. > > > > > >> Currently > > > > > >> Tx on non-zero-copy interfaces is synchronous, and that doesn't > > > > > >> allow to > > > > > >> scale well. > > > > > > > > > > > > Interestingly, actually, there are a lot of "duplication" between > > > > > > io_uring and AF_XDP: > > > > > > > > > > > > 1) both have similar memory model (user register) > > > > > > 2) both use ring for communication > > > > > > > > > > > > I wonder if we can let io_uring talks directly to AF_XDP. > > > > > > > > > > Well, if we submit poll() in QEMU main loop via io_uring, then we can > > > > > avoid cost of the synchronous Tx for non-zero-copy modes, i.e. for > > > > > virtual interfaces. io_uring thread in the kernel will be able to > > > > > perform transmission for us. > > > > > > > > It would be nice if we can use iothread/vhost other than the main loop > > > > even if io_uring can use kthreads. We can avoid the memory translation > > > > cost. > > > > > > The QEMU event loop (AioContext) has io_uring code > > > (utils/fdmon-io_uring.c) but it's disabled at the moment. I'm working > > > on patches to re-enable it and will probably send them in July. The > > > patches also add an API to submit arbitrary io_uring operations so > > > that you can do stuff besides file descriptor monitoring. Both the > > > main loop and IOThreads will be able to use io_uring on Linux hosts. > > > > Just to make sure I understand. If we still need a copy from guest to > > io_uring buffer, we still need to go via memory API for GPA which > > seems expensive. > > > > Vhost seems to be a shortcut for this. > > I'm not sure how exactly you're thinking of using io_uring. > > Simply using io_uring for the event loop (file descriptor monitoring) > doesn't involve an extra buffer, but the packet payload still needs to > reside in AF_XDP umem, so there is a copy between guest memory and > umem.
So there would be a translation from GPA to HVA (unless io_uring support 2 stages) which needs to go via qemu memory core. And this part seems to be very expensive according to my test in the past. > If umem encompasses guest memory, It requires you to pin the whole guest memory and a GPA to HVA translation is still required. Thanks >it may be possible to avoid > copying the packet payload. > > Stefan >