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

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Date: Mon, 17 Jun 2024 14:28:05 +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 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.

Thanks