netdev - Re: [PATCH net-next] xsk: introduce xsk_dma

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Date:   Tue, 18 Apr 2023 09:07:30 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        Christoph Hellwig <hch@...radead.org>, netdev@...r.kernel.org,
        Björn Töpel <bjorn@...nel.org>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        "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>, bpf@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Gerd Hoffmann <kraxel@...hat.com>
Subject: Re: [PATCH net-next] xsk: introduce xsk_dma_ops

On Tue, Apr 18, 2023 at 2:58 AM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Mon, 17 Apr 2023 11:56:10 -0700 Jakub Kicinski wrote:
> > > May misunderstand, here the "dma_ops" is not the "dma_ops" of DMA API.
> > >
> > > I mean the callbacks for xsk to do dma.
> > >
> > > Maybe, I should rename it in the next version.
> >
> > Would you mind explaining this a bit more to folks like me who are not
> > familiar with VirtIO?  DMA API is supposed to hide the DMA mapping
> > details from the stack, why is it not sufficient here.

The reason is that legacy virtio device don't use DMA(vring_use_dma_api()).

The AF_XDP assumes DMA for netdev doesn't work in this case. We need a
way to make it work.

Thanks

>
> Umm.. also it'd help to post the user of the API in the same series.
> I only see the XSK changes, maybe if the virtio changes were in
> the same series I could answer my own question.
>