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

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Date:   Mon, 17 Apr 2023 23:19:47 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, 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>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH net-next] xsk: introduce xsk_dma_ops

On Mon, 17 Apr 2023 22:01:36 -0700 Christoph Hellwig wrote:
> 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.

What bus are the classic s390 on (in terms of the device model)?

> > 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.

Damn, that's unfortunate. Thinking aloud -- that means that if we want 
to continue to pull memory management out of networking drivers to
improve it for all, cross-optimize with the rest of the stack and
allow various upcoming forms of zero copy -- then we need to add an
equivalent of dma_ops and DMA API locally in networking?