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

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Date: Mon, 31 Jul 2023 08:46:51 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Jason Wang <jasowang@...hat.com>
Cc: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, 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>
Subject: Re: [PATCH vhost v11 05/10] virtio_ring: introduce
 virtqueue_dma_dev()

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?

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