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Date:   Mon, 17 Apr 2023 19:54:00 -0700
From:   Jakub Kicinski <kuba@kernel.org>
To:     Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Cc:     Christoph Hellwig <hch@infradead.org>, netdev@vger.kernel.org,
        =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn@kernel.org>,
        Magnus Karlsson <magnus.karlsson@intel.com>,
        Maciej Fijalkowski <maciej.fijalkowski@intel.com>,
        Jonathan Lemon <jonathan.lemon@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Paolo Abeni <pabeni@redhat.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        John Fastabend <john.fastabend@gmail.com>, bpf@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Guenter Roeck <linux@roeck-us.net>,
        Gerd Hoffmann <kraxel@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jens Axboe <axboe@kernel.dk>
Subject: Re: [PATCH net-next] xsk: introduce xsk_dma_ops
Message-ID: <20230417195400.482cfe75@kernel.org>
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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.