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

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Message-ID: <20210920110216.4c54c9a3@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Mon, 20 Sep 2021 11:02:16 -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 Sat, 18 Sep 2021 13:53:35 +0200 Toke Høiland-Jørgensen wrote:
> I'm OK with a bpf_header_pointer()-type helper - I quite like the
> in-kernel version of this for SKBs, so replicating it as a BPF helper
> would be great. But I'm a little worried about taking a performance hit.
> 
> I.e., if you do:
> 
> ptr = bpf_header_pointer(pkt, offset, len, stack_ptr)
> *ptr = xxx;
> 
> then, if the helper ended up copying the data into the stack pointer,
> you didn't actually change anything in the packet, so you need to do a
> writeback.
> 
> Jakub suggested up-thread that this should be done with some kind of
> flush() helper. But you don't know whether the header_pointer()-helper
> copied the data, so you always need to call the flush() helper, which
> will incur overhead. If the verifier can in-line the helpers that will
> lower it, but will it be enough to make it negligible?

Depends on the assumptions the program otherwise makes, right?

For reading I'd expect a *layout-independent* TC program would 
replace approximately:

ptr = <some_ptr>;
if (ptr + CONST >= md->ptr_end)
	if (bpf_pull_data(md, off + CONST))
		return DROP;
	ptr = <some_ptr>;
	if (ptr + CONST >= md->ptr_end)
		return DROP; /* da hell? */
}

With this (pre-inlining):

ptr = bpf_header_pointer(md, offset, len, stack);
if (!ptr)
	return DROP;

Post-inlining (assuming static validation of args to prevent wraps):

if (md->ptr + args->off + args->len < md->ptr_end)
	ptr = md->ptr + args->off;
else
	ptr = __bpf_header_pointer(md, offset, len, stack);
if (!ptr)
	return DROP;

But that's based on guesswork so perhaps I'm off base.


Regarding the flush() I was expecting that most progs will not modify
the packet (or at least won't modify most headers they load) so no
point paying the price of tracking changes auto-magically.

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)
		bpf_header_pointer_flush();


is simple enough.. to me.