netdev - Re: MSG_ZEROCOPY

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Date:   Wed, 11 Nov 2020 00:20:22 +0000
From:   Victor Stewart <v@...etag.social>
To:     Jonathan Lemon <jonathan.lemon@...il.com>
Cc:     netdev@...r.kernel.org
Subject: Re: MSG_ZEROCOPY_FIXED

On Wed, Nov 11, 2020 at 12:09 AM Jonathan Lemon
<jonathan.lemon@...il.com> wrote:
>
> On Sun, Nov 08, 2020 at 05:04:41PM +0000, Victor Stewart wrote:
> > hi all,
> >
> > i'm seeking input / comment on the idea of implementing full fledged
> > zerocopy UDP networking that uses persistent buffers allocated in
> > userspace... before I go off on a solo tangent with my first patches
> > lol.
> >
> > i'm sure there's been lots of thought/discussion on this before. of
> > course Willem added MSG_ZEROCOPY on the send path (pin buffers on
> > demand / per send). and something similar to what I speak of exists
> > with TCP_ZEROCOPY_RECEIVE.
> >
> > i envision something like a new flag like MSG_ZEROCOPY_FIXED that
> > "does the right thing" in the send vs recv paths.
>
> See the netgpu patches that I posted earlier; these will handle
> protocol independent zerocopy sends/receives.  I do have a working
> UDP receive implementation which will be posted with an updated
> patchset.

amazing i'll check it out. thanks.

does your udp zerocopy receive use mmap-ed buffers then vm_insert_pfn
/ remap_pfn_range to remap the physical pages of the received payload
into the memory submitted by recvmsg for reception?

https://lore.kernel.org/io-uring/acc66238-0d27-cd22-dac4-928777a8efbc@gmail.com/T/#t

^^ and check the thread from today on the io_uring mailing list going
into the mechanics of zerocopy sendmsg i have in mind.

(TLDR; i think it should be io_uring "only" so that we can collapse it
into a single completion event, aka when the NIC ACKs the
transmission. and exploiting the asynchrony of io_uring is the only
way to do this? so you'd submit your sendmsg operation to io_uring and
instead of receiving a completion event when the send gets enqueued,
you'd only get it upon failure or NIC ACK).

> --
> Jonathan