netdev - Re: [PATCH RFC v2 00/12] socket sendmsg MSG

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Message-ID: <1488339791.9415.285.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Tue, 28 Feb 2017 19:43:11 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     David Miller <davem@...emloft.net>
Cc:     luto@...capital.net, willemdebruijn.kernel@...il.com,
        mtk.manpages@...il.com, netdev@...r.kernel.org, willemb@...gle.com,
        linux-api@...r.kernel.org
Subject: Re: [PATCH RFC v2 00/12] socket sendmsg MSG_ZEROCOPY

On Tue, 2017-02-28 at 22:28 -0500, David Miller wrote:

> These device are already choking, because as I stated this can already
> be done via sendfile().
> 
> Networking card wise this isn't an issue, chips bring the entire packet
> into their FIFO, compute checksums on the fly mid-stream, and then write
> the 16-bit checksum field before starting to write the packet onto the
> wire.
> 
> I think this is completely a non-issue, and we thought about this right
> from the start when sendfile() support was added nearly two decades ago.
> If network cards from back then didn't crap out in this situation I
> think the ones out there now are probably ok.

Well, we had to fix one issue with GSO fall back about 4 years ago.

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=cef401de7be8c4e155c6746bfccf721a4fa5fab9

So extra scrutiny would be nice.

Zero copy is incredibly hard to get right.