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

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Date:   Mon, 17 Apr 2023 19:54:00 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc:     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>,
        Jason Wang <jasowang@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH net-next] xsk: introduce xsk_dma_ops

On Tue, 18 Apr 2023 10:19:39 +0800 Xuan Zhuo wrote:
> > Can we not push this down to be bus level? virtio has its own bus it
> > can plug in whatever magic it wants into dma ops.  
> 
> It is actually not possible.
> 
> [1] https://lore.kernel.org/virtualization/ZDUCDeYLqAwQVJe7@infradead.org/

Maybe Christoph, or Greg can comment.

AF_XDP, io_uring, and increasing number of pinned memory / zero copy
implementations need to do DMA mapping outside the drivers.
I don't think it's reasonable to be bubbling up custom per-subsystem
DMA ops into all of them for the sake of virtio.

> > Otherwise it really seems like we're bubbling up a virtio hack into
> > generic code :(  
> 
> Can we understand the purpose of this matter to back the DMA operation to the
> driver?

We understand what your code does.