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

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Date:   Mon, 20 Sep 2021 23:01:48 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Jakub Kicinski <kuba@...nel.org>
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

Jakub Kicinski <kuba@...nel.org> writes:

> 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.

Yeah, that's more or less what I was thinking...

> 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.

... but I guess my mental model assumed that packet writes would be just
as frequent as reads, in which case it would be a win if you could amend
your example like:

> 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?

-Toke

> 		bpf_header_pointer_flush();
>
>
> is simple enough.. to me.