netdev - Re: [PATCH net-next v24 11/13] net: add devmem TCP documentation

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Message-ID: <20240903145150.4ffee51b@kernel.org>
Date: Tue, 3 Sep 2024 14:51:50 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Mina Almasry <almasrymina@...gle.com>
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Subject: Re: [PATCH net-next v24 11/13] net: add devmem TCP documentation

On Sat, 31 Aug 2024 00:43:11 +0000 Mina Almasry wrote:
> +The socket must be flow steered to the dmabuf bound RX queue::
> +
> +	ethtool -N eth1 flow-type tcp4 ... queue 15,

nit: tailing comma here

> +Devmem payloads are inaccessible to the kernel processing the packets. This
> +results in a few quirks for payloads of devmem skbs:
> +
> +- Loopback is not functional. Loopback relies on copying the payload, which is
> +  not possible with devmem skbs.
> +
> +- Software checksum calculation fails.

Speaking of which, I think we need to add readability check to 
skb_checksum_help(). Shouldn't the check in __skb_checksum() 
have a WARN_ON_ONCE() around it? It's impossible to return 
an error from there.

> +- TCP Dump and bpf can't access devmem packet payloads.
> +
> +
> +Testing
> +=======
> +
> +More realistic example code can be found in the kernel source under
> +tools/testing/selftests/net/ncdevmem.c

looks like HTML output wraps the file path, maybe quote it as
``tools/testing/selftests/net/ncdevmem.c`` ?