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

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Date:   Wed, 19 Apr 2023 23:19:22 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        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 Wed, Apr 19, 2023 at 09:45:06AM -0700, Jakub Kicinski wrote:
> > Can you explain what the actual use case is?
> > 
> > From the original patchset I suspect it is dma mapping something very
> > long term and then maybe doing syncs on it as needed?
> 
> In this case yes, pinned user memory, it gets sliced up into MTU sized
> chunks, fed into an Rx queue of a device, and user can see packets
> without any copies.

How long is the life time of these mappings?  Because dma_map_*
assumes a temporary mapping and not one that is pinned bascically
forever.

> Quite similar use case #2 is upcoming io_uring / "direct placement"
> patches (former from Meta, latter for Google) which will try to receive
> just the TCP data into pinned user memory.

I don't think we can just long term pin user memory here.  E.g. for
confidential computing cases we can't even ever do DMA straight to
userspace.  I had that conversation with Meta's block folks who
want to do something similar with io_uring and the only option is an
an allocator for memory that is known DMAable, e.g. through dma-bufs.

You guys really all need to get together and come up with a scheme
that actually works instead of piling these hacks over hacks.