On 7/7/23 03:43, Jason Wang 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.
So, we can have a thread running as part of QEMU process that implements vhost functionality for a virtio-net device. And this thread has an optimized way to access memory. What prevents current virtio-net emulation code accessing memory in the same optimized way? i.e. we likely don't actually need to implement the whole vhost-virtio communication protocol in order to have faster memory access from the device emulation code. I mean, if vhost can access device memory faster, why device itself can't? With that we could probably split the "datapath" part of the virtio-net emulation into a separate thread driven by iothread loop. Then add batch API for communication with a network backend (af-xdp) to avoid per-packet calls. These are 3 more or less independent tasks that should allow the similar performance to a full fledged vhost control and dataplane implementation inside QEMU. Or am I missing something? (Probably) > > Thanks > >> >>> >>> 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 >>>> >>> >> >
