netdev - Re: [PATCH net-next v5 08/15] virtio_net: sq support premapped mode

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Date: Tue, 18 Jun 2024 09:00:56 +0800
From: Jason Wang <jasowang@...hat.com>
To: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc: netdev@...r.kernel.org, "Michael S. Tsirkin" <mst@...hat.com>, 
	Eugenio Pérez <eperezma@...hat.com>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, 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>, virtualization@...ts.linux.dev, 
	bpf@...r.kernel.org
Subject: Re: [PATCH net-next v5 08/15] virtio_net: sq support premapped mode

On Mon, Jun 17, 2024 at 3:41 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
>
> On Mon, 17 Jun 2024 14:28:05 +0800, Jason Wang <jasowang@...hat.com> wrote:
> > On Mon, Jun 17, 2024 at 1:00 PM Jason Wang <jasowang@...hat.com> wrote:
> > >
> > > On Fri, Jun 14, 2024 at 2:39 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
> > > >
> > > > If the xsk is enabling, the xsk tx will share the send queue.
> > > > But the xsk requires that the send queue use the premapped mode.
> > > > So the send queue must support premapped mode when it is bound to
> > > > af-xdp.
> > > >
> > > > * virtnet_sq_set_premapped(sq, true) is used to enable premapped mode.
> > > >
> > > >     In this mode, the driver will record the dma info when skb or xdp
> > > >     frame is sent.
> > > >
> > > >     Currently, the SQ premapped mode is operational only with af-xdp. In
> > > >     this mode, af-xdp, the kernel stack, and xdp tx/redirect will share
> > > >     the same SQ. Af-xdp independently manages its DMA. The kernel stack
> > > >     and xdp tx/redirect utilize this DMA metadata to manage the DMA
> > > >     info.
> > > >
> >
> > Note that there's indeed a mode when we have exclusive XDP TX queue:
> >
> >         /* XDP requires extra queues for XDP_TX */
> >         if (curr_qp + xdp_qp > vi->max_queue_pairs) {
> >                 netdev_warn_once(dev, "XDP request %i queues but max
> > is %i. XDP_TX and XDP_REDIRECT will operate in a slower locked tx
> > mode.\n",
> >                                  curr_qp + xdp_qp, vi->max_queue_pairs);
> >                 xdp_qp = 0;
> >         }
> >
> > So we need to mention how the code works in this patch.
>
> Sorry, I do not get it.
>
> Could you say more?

I meant in the commit log, you said:

"""
In this mode, af-xdp, the kernel stack, and xdp tx/redirect will share
the same SQ.
"""

is not correct if we have sufficient queue pairs.

We need to tweak it and explain if the code can still work if we have
exclusive XDP TX queues.

Thanks

>
> Thanks.
>
>
> >
> > Thanks
> >
>