netdev - Re: [PATCH bpf] xsk: mark napi

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Date:   Thu, 30 Jun 2022 08:52:52 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Cc:     Björn Töpel <bjorn.topel@...il.com>,
        bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Netdev <netdev@...r.kernel.org>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>
Subject: Re: [PATCH bpf] xsk: mark napi_id on sendmsg()

On Thu, 30 Jun 2022 13:53:11 +0200 Maciej Fijalkowski wrote:
> > > Would it be possible to move the marking to when the queue is getting
> > > bound instead of the recv/send paths?   
> > 
> > I mean when socket is getting bound.  
> 
> So Bjorn said that it was the design choice to follow the standard
> sockets' approach. I'm including a dirty diff for a discussion which
> allows me to get napi_id at bind() time. But, this works for ice as this
> driver during the XDP prog/XSK pool load only disables the NAPI, so we are
> sure that napi_id stays the same. That might not be the case for other
> AF_XDP ZC enabled drivers though, they might delete the NAPI and this
> approach wouldn't work...or am I missing something?

Possible, but IDK if we're changing anything substantially in that
regard. The existing code already uses sk_mark_napi_id_once() so
if the NAPI ID changes during the lifetime of the socket the user
will be out of luck already. But no strong feelings.

> I'd prefer the diff below though as it simplifies the data path, but I
> can't say if it's safe to do so. We would have to be sure about drivers
> keeping their NAPI struct. This would also allow us to drop napi_id from
> xdp_rxq_info.