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

[Jason Wang <jasowang@redhat.com>]

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 :)

It's something like linking SPDK/DPDK to 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
>