netdev - How much is checksumming done in the kernel vs on the NIC?

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Message-ID: <1015189.1746187621@warthog.procyon.org.uk>
Date: Fri, 02 May 2025 13:07:01 +0100
From: David Howells <dhowells@...hat.com>
To: "David S. Miller" <davem@...emloft.net>,
    Jakub Kicinski <kuba@...nel.org>
cc: dhowells@...hat.com, willy@...radead.org, netdev@...r.kernel.org
Subject: How much is checksumming done in the kernel vs on the NIC?

Hi Dave, Jakub,

I'm looking into making the sendmsg() code properly handle the 'DIO vs fork'
issue (where pages need pinning rather than refs taken) and also getting rid
of the taking of refs entirely as the page refcount is going to go away in the
relatively near future.

I'm wondering quite how to do the approach, and I was wondering if you have
any idea about the following:

 (1) How much do we need to do packet checksumming in the kernel these days
     rather than offloading it to the NIC?

 (2) How often do modern kernels encounter NICs that can only take a single
     {pointer,len} extent for any particular packet rather than a list of
     such?

Thanks,
David