netdev - Re: [PATCH vhost v11 05/10] virtio_ring: introduce virtqueue_dma

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Message-ID: <1690855424.7821567-1-xuanzhuo@linux.alibaba.com>
Date: Tue, 1 Aug 2023 10:03:44 +0800
From: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Christoph Hellwig <hch@...radead.org>,
 virtualization@...ts.linux-foundation.org,
 "David S.  Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>,
 Paolo  Abeni <pabeni@...hat.com>,
 Alexei Starovoitov <ast@...nel.org>,
 Daniel  Borkmann <daniel@...earbox.net>,
 Jesper Dangaard Brouer <hawk@...nel.org>,
 John Fastabend <john.fastabend@...il.com>,
 netdev@...r.kernel.org,
 bpf@...r.kernel.org,
 "Michael S. Tsirkin" <mst@...hat.com>,
 Jason Wang <jasowang@...hat.com>
Subject: Re: [PATCH vhost v11 05/10] virtio_ring: introduce virtqueue_dma_dev()

On Mon, 31 Jul 2023 08:46:51 -0700, Jakub Kicinski <kuba@...nel.org> wrote:
> On Mon, 31 Jul 2023 09:23:29 +0800 Jason Wang wrote:
> > > I'd step back and ask you why do you want to use AF_XDP with virtio.
> > > Instead of bifurcating one virtio instance into different queues why
> > > not create a separate virtio instance?
> >
> > I'm not sure I get this, but do you mean a separate virtio device that
> > owns AF_XDP queues only? If I understand it correctly, bifurcating is
> > one of the key advantages of AF_XDP. What's more, current virtio
> > doesn't support being split at queue (pair) level. And it may still
> > suffer from the yes/no DMA API issue.
>
> I guess we should step even further back and ask Xuan what the use case
> is, because I'm not very sure. All we hear is "enable AF_XDP on virtio"
> but AF_XDP is barely used on real HW, so why?

Why just for real HW?

I want to enable AF_XDP on virtio-net. Then the user can send/recv packets by
AF_XDP bypass through the kernel. That has be used on large scale.

I donot know what is the problem of the virtio-net.
Why do you think that the virtio-net cannot work with AF_XDP?

>
> Bifurcating makes (used to make?) some sense in case of real HW when you
> had only one PCI function and had to subdivide it.

Sorry I do not get this.

> Virtio is either a SW
> construct or offloaded to very capable HW, so either way cost of
> creating an extra instance for DPDK or whatever else is very low.

The extra instance is virtio-net?

I think there is a gap. So let me give you a brief introduction of our case.

Firstly, we donot use dpdk. We use the AF_XDP, because of that the AF_XDP is
more simpler and easy to deploy for the nginx.

We use the AF_XDP to speedup the UDP of the quic. By the library, the APP just
needs some simple change.

On the AliYun, the net driver is virtio-net. So we want the virtio-net support
the AF_XDP.

I guess what you mean is that we can speed up through the cooperation of devices
and drivers, but our machines are public clouds, and we cannot change the
back-end devices of virtio under normal circumstances.

Here I do not know the different of the real hw and the virtio-net.

Thanks.