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

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Message-ID: <1681981908.9700203-3-xuanzhuo@linux.alibaba.com>
Date:   Thu, 20 Apr 2023 17:11:48 +0800
From:   Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
To:     Christoph Hellwig <hch@...radead.org>
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>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH net-next] xsk: introduce xsk_dma_ops

On Wed, 19 Apr 2023 23:19:22 -0700, Christoph Hellwig <hch@...radead.org> wrote:
> 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.

I think that cases Jakub mentioned are new requirements. From the perspective of
implementation, compared to this patch I submitted, if the DMA API can be
expanded, compatible with some special hardware, I think it is a good solution.

I know that the current design of DMA API only supports some physical hardware,
but can it be modified or expanded?

Still the previous idea, can we add a new ops (not dma_ops) in device? After the
driver configuration, so that the DMA API can be compatible with some special
hardware?


Thanks.