netdev - Re: [PATCH v14 bpf-next 00/18] mvneta: introduce XDP multi-buffer support

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Date:   Mon, 20 Sep 2021 14:25:42 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     John Fastabend <john.fastabend@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Lorenzo Bianconi <lorenzo.bianconi@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, shayagr@...zon.com,
        David Ahern <dsahern@...nel.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Eelco Chaudron <echaudro@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Saeed Mahameed <saeed@...nel.org>,
        "Fijalkowski, Maciej" <maciej.fijalkowski@...el.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        tirthendu.sarkar@...el.com
Subject: Re: [PATCH v14 bpf-next 00/18] mvneta: introduce XDP multi-buffer
 support

On Mon, 20 Sep 2021 23:01:48 +0200 Toke Høiland-Jørgensen wrote:
> > In fact I don't think there is anything infra can do better for
> > flushing than the prog itself:
> >
> > 	bool mod = false;
> >
> > 	ptr = bpf_header_pointer(...);
> > 	...
> > 	if (some_cond(...)) {
> > 		change_packet(...);
> > 		mod = true;
> > 	}
> > 	...
> > 	if (mod)  
> 
> to have an additional check like:
> 
> if (mod && ptr == stack)
> 
> (or something to that effect). No?

Good point. Do you think we should have the kernel add/inline this
optimization or have the user do it explicitly.

The draft API was:

void *xdp_mb_pointer_flush(struct xdp_buff *xdp_md, u32 flags, 
                           u32 offset, u32 len, void *stack_buf)

Which does not take the ptr returned by header_pointer(), but that's
easy to add (well, easy other than the fact it'd be the 6th arg).

BTW I drafted the API this way to cater to the case where flush()
is called without a prior call to header_pointer(). For when packet
trailer or header is populated directly from a map value. Dunno if
that's actually useful, either.