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

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Date:   Fri, 21 Apr 2023 15:31:04 +0800
From:   Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     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>,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH net-next] xsk: introduce xsk_dma_ops

On Thu, 20 Apr 2023 07:13:49 -0700, Jakub Kicinski <kuba@...nel.org> wrote:
> On Wed, 19 Apr 2023 23:19:22 -0700 Christoph Hellwig wrote:
> > > 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.
>
> Yeah, this one is "for ever".
>
> > > 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.
>
> Okay, that simplifies various aspects. We'll just used dma-bufs from
> the start in the new APIs.


I am not particularly familiar with dma-bufs. I want to know if this mechanism
can solve the problem of virtio-net.

I saw this framework, allowing the driver do something inside the ops of
dma-bufs.

If so, is it possible to propose a new patch based on dma-bufs?

Thanks.