netdev - Re: Redux: Backwards compatibility for XDP multi-buff

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Message-ID: <20210921155118.439c0aa9@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Tue, 21 Sep 2021 15:51:18 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Zvi Effron <zeffron@...tgames.com>,
        Lorenz Bauer <lmb@...udflare.com>,
        Lorenzo Bianconi <lbianconi@...hat.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>
Subject: Re: Redux: Backwards compatibility for XDP multi-buff

On Wed, 22 Sep 2021 00:20:19 +0200 Toke Høiland-Jørgensen wrote:
> >> Neither of those are desirable outcomes, I think; and if we add a
> >> separate "XDP multi-buff" switch, we might as well make it system-wide?  
> >
> > If we have an internal flag 'this driver supports multi-buf xdp' cannot we
> > make xdp_redirect to linearize in case the packet is being redirected
> > to non multi-buf aware driver (potentially with corresponding non mb aware xdp
> > progs attached) from mb aware driver?  
> 
> Hmm, the assumption that XDP frames take up at most one page has been
> fundamental from the start of XDP. So what does linearise mean in this
> context? If we get a 9k packet, should we dynamically allocate a
> multi-page chunk of contiguous memory and copy the frame into that, or
> were you thinking something else?

My $.02 would be to not care about redirect at all.

It's not like the user experience with redirect is anywhere close 
to amazing right now. Besides (with the exception of SW devices which
will likely gain mb support quickly) mixed-HW setups are very rare.
If the source of the redirect supports mb so will likely the target.