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. > > It's something like linking SPDK/DPDK to Qemu. Sergio Lopez tried loading vhost-user devices as shared libraries that run in the QEMU process. It worked as an experiment but wasn't pursued further. I think that might make sense in specific cases where there is an existing vhost-user codebase that needs to run as part of QEMU. In this case the AF_XDP code is new, so it's not a case of moving existing code into QEMU. > > > > > > > > > Regarding pinning - I wonder if that's something that can be refined > > > > > > in the kernel by adding an AF_XDP flag that enables on-demand > > > > > > pinning > > > > > > of umem. That way only rx and tx buffers that are currently in use > > > > > > will be pinned. The disadvantage is the runtime overhead to > > > > > > pin/unpin > > > > > > pages. I'm not sure whether it's possible to implement this, I > > > > > > haven't > > > > > > checked the kernel code. > > > > > > > > > > It requires the device to do page faults which is not commonly > > > > > supported nowadays. > > > > > > > > I don't understand this comment. AF_XDP processes each rx/tx > > > > descriptor. At that point it can getuserpages() or similar in order to > > > > pin the page. When the memory is no longer needed, it can put those > > > > pages. No fault mechanism is needed. What am I missing? > > > > > > Ok, I think I kind of get you, you mean doing pinning while processing > > > rx/tx buffers? It's not easy since GUP itself is not very fast, it may > > > hit PPS for sure. > > > > Yes. It's not as fast as permanently pinning rx/tx buffers, but it > > supports unpinned guest RAM. > > Right, it's a balance between pin and PPS. PPS seems to be more > important in this case. > > > > > There are variations on this approach, like keeping a certain amount > > of pages pinned after they have been used so the cost of > > pinning/unpinning can be avoided when the same pages are reused in the > > future, but I don't know how effective that is in practice. > > > > Is there a more efficient approach without relying on hardware page > > fault support? > > I guess so, I see some slides that say device page fault is very slow. > > > > > My understanding is that hardware page fault support is not yet > > deployed. We'd be left with pinning guest RAM permanently or using a > > runtime pinning/unpinning approach like I've described. > > Probably. > > Thanks > > > > > Stefan > > >
