Re: [PATCH] net: add initial support for AF_XDP network backend

[Jason Wang]

On Mon, Jul 10, 2023 at 11:14 PM Stefan Hajnoczi <stefanha@gmail.com> wrote:
>
> On Thu, 6 Jul 2023 at 21:43, Jason Wang <jasowang@redhat.com> wrote:
> >
> > On Fri, Jul 7, 2023 at 3:08 AM Stefan Hajnoczi <stefanha@gmail.com> wrote:
> > >
> > > On Wed, 5 Jul 2023 at 02:02, Jason Wang <jasowang@redhat.com> wrote:
> > > >
> > > > On Mon, Jul 3, 2023 at 5:03 PM Stefan Hajnoczi <stefanha@gmail.com> 
> > > > wrote:
> > > > >
> > > > > On Fri, 30 Jun 2023 at 09:41, Jason Wang <jasowang@redhat.com> wrote:
> > > > > >
> > > > > > On Thu, Jun 29, 2023 at 8:36 PM Stefan Hajnoczi 
> > > > > > <stefanha@gmail.com> wrote:
> > > > > > >
> > > > > > > On Thu, 29 Jun 2023 at 07:26, Jason Wang <jasowang@redhat.com> 
> > > > > > > wrote:
> > > > > > > >
> > > > > > > > On Wed, Jun 28, 2023 at 4:25 PM Stefan Hajnoczi 
> > > > > > > > <stefanha@gmail.com> wrote:
> > > > > > > > >
> > > > > > > > > On Wed, 28 Jun 2023 at 10:19, Jason Wang 
> > > > > > > > > <jasowang@redhat.com> wrote:
> > > > > > > > > >
> > > > > > > > > > On Wed, Jun 28, 2023 at 4:15 PM Stefan Hajnoczi 
> > > > > > > > > > <stefanha@gmail.com> wrote:
> > > > > > > > > > >
> > > > > > > > > > > On Wed, 28 Jun 2023 at 09:59, Jason Wang 
> > > > > > > > > > > <jasowang@redhat.com> wrote:
> > > > > > > > > > > >
> > > > > > > > > > > > On Wed, Jun 28, 2023 at 3:46 PM Stefan Hajnoczi 
> > > > > > > > > > > > <stefanha@gmail.com> wrote:
> > > > > > > > > > > > >
> > > > > > > > > > > > > On Wed, 28 Jun 2023 at 05:28, Jason Wang 
> > > > > > > > > > > > > <jasowang@redhat.com> wrote:
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > On Wed, Jun 28, 2023 at 6:45 AM Ilya Maximets 
> > > > > > > > > > > > > > <i.maximets@ovn.org> wrote:
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > On 6/27/23 04:54, Jason Wang wrote:
> > > > > > > > > > > > > > > > On Mon, Jun 26, 2023 at 9:17 PM Ilya Maximets 
> > > > > > > > > > > > > > > > <i.maximets@ovn.org> wrote:
> > > > > > > > > > > > > > > >>
> > > > > > > > > > > > > > > >> On 6/26/23 08:32, Jason Wang wrote:
> > > > > > > > > > > > > > > >>> On Sun, Jun 25, 2023 at 3:06 PM Jason Wang 
> > > > > > > > > > > > > > > >>> <jasowang@redhat.com> wrote:
> > > > > > > > > > > > > > > >>>>
> > > > > > > > > > > > > > > >>>> On Fri, Jun 23, 2023 at 5:58 AM Ilya 
> > > > > > > > > > > > > > > >>>> Maximets <i.maximets@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.

Probably.

>
> 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.

Yes, it can do what vhost can do. Starting from a vhost may help us to
know where we could go for the optimization of the memory core.

Thanks

>
> Stefan
>