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

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Date:   Mon, 17 Apr 2023 22:01:36 -0700
From:   Christoph Hellwig <hch@...radead.org>
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>,
        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 Mon, Apr 17, 2023 at 07:54:00PM -0700, Jakub Kicinski wrote:
> AF_XDP, io_uring, and increasing number of pinned memory / zero copy
> implementations need to do DMA mapping outside the drivers.

You can't just do dma mapping outside the driver, because there are
drivers that do not require DMA mapping at all.  virtio is an example,
but all the classic s390 drivers and some other odd virtualization
ones are others.

> 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.

dma addresses and thus dma mappings are completely driver specific.
Upper layers have no business looking at them.