On Thu, 6 Jul 2023 at 21:43, Jason Wang <jasow...@redhat.com> wrote: > > On Fri, Jul 7, 2023 at 3:08 AM Stefan Hajnoczi <stefa...@gmail.com> wrote: > > > > On Wed, 5 Jul 2023 at 02:02, Jason Wang <jasow...@redhat.com> wrote: > > > > > > On Mon, Jul 3, 2023 at 5:03 PM Stefan Hajnoczi <stefa...@gmail.com> wrote: > > > > > > > > On Fri, 30 Jun 2023 at 09:41, Jason Wang <jasow...@redhat.com> wrote: > > > > > > > > > > On Thu, Jun 29, 2023 at 8:36 PM Stefan Hajnoczi <stefa...@gmail.com> > > > > > wrote: > > > > > > > > > > > > On Thu, 29 Jun 2023 at 07:26, Jason Wang <jasow...@redhat.com> > > > > > > wrote: > > > > > > > > > > > > > > On Wed, Jun 28, 2023 at 4:25 PM Stefan Hajnoczi > > > > > > > <stefa...@gmail.com> wrote: > > > > > > > > > > > > > > > > On Wed, 28 Jun 2023 at 10:19, Jason Wang <jasow...@redhat.com> > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > 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. > > > > > > > > > > > > > > > > Yes, but in the current approach where AF_XDP is implemented as > > > > > > > > a QEMU > > > > > > > > netdev, there is already QEMU device emulation (e.g. virtio-net) > > > > > > > > happening. So the GPA to HVA translation will happen anyway in > > > > > > > > device > > > > > > > > emulation. > > > > > > > > > > > > > > Just to make sure we're on the same page. > > > > > > > > > > > > > > I meant, AF_XDP can do more than e.g 10Mpps. So if we still use > > > > > > > the > > > > > > > QEMU netdev, it would be very hard to achieve that if we stick to > > > > > > > using the Qemu memory core translations which need to take care > > > > > > > about > > > > > > > too much extra stuff. That's why I suggest using vhost in io > > > > > > > threads > > > > > > > which only cares about ram so the translation could be very fast. > > > > > > > > > > > > What does using "vhost in io threads" mean? > > > > > > > > > > It means a vhost userspace dataplane that is implemented via io > > > > > threads. > > > > > > > > AFAIK this does not exist today. QEMU's built-in devices that use > > > > IOThreads don't use vhost code. QEMU vhost code is for vhost kernel, > > > > vhost-user, or vDPA but not built-in devices that use IOThreads. The > > > > built-in devices implement VirtioDeviceClass callbacks directly and > > > > use AioContext APIs to run in IOThreads. > > > > > > Yes. > > > > > > > > > > > Do you have an idea for using vhost code for built-in devices? Maybe > > > > it's fastest if you explain your idea and its advantages instead of me > > > > guessing. > > > > > > It's something like I'd proposed in [1]: > > > > > > 1) a vhost that is implemented via IOThreads > > > 2) memory translation is done via vhost memory table/IOTLB > > > > > > The advantages are: > > > > > > 1) No 3rd application like DPDK application > > > 2) Attack surface were reduced > > > 3) Better understanding/interactions with device model for things like > > > RSS and IOMMU > > > > > > There could be some dis-advantages but it's not obvious to me :) > > > > Why is QEMU's native device emulation API not the natural choice for > > writing built-in devices? I don't understand why the vhost interface > > is desirable for built-in devices. > > Unless the memory helpers (like address translations) were optimized > fully to satisfy this 10M+ PPS. > > Not sure if this is too hard, but last time I benchmark, perf told me > most of the time spent in the translation. > > Using a vhost is a workaround since its memory model is much more > simpler so it can skip lots of memory sections like I/O and ROM etc.
I see, that sounds like a question of optimization. Most DMA transfers will be to/from guest RAM and it seems like QEMU's memory API could be optimized for that case. PIO/MMIO dispatch could use a different API from DMA transfers, if necessary. I don't think there is a fundamental reason why QEMU's own device emulation code cannot translate memory as fast as vhost devices can. Stefan
