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

From: Eelco Chaudron <echaudro@redhat.com>
To: "Toke Høiland-Jørgensen" <toke@redhat.com>
Cc: Jakub Kicinski <kuba@kernel.org>,
	John Fastabend <john.fastabend@gmail.com>,
	Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	Lorenzo Bianconi <lorenzo@kernel.org>, bpf <bpf@vger.kernel.org>,
	Network Development <netdev@vger.kernel.org>,
	Lorenzo Bianconi <lorenzo.bianconi@redhat.com>,
	"David S. Miller" <davem@davemloft.net>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	shayagr@amazon.com, David Ahern <dsahern@kernel.org>,
	Jesper Dangaard Brouer <brouer@redhat.com>,
	Jason Wang <jasowang@redhat.com>,
	Alexander Duyck <alexander.duyck@gmail.com>,
	Saeed Mahameed <saeed@kernel.org>,
	"Fijalkowski, Maciej" <maciej.fijalkowski@intel.com>,
	"Karlsson, Magnus" <magnus.karlsson@intel.com>,
	tirthendu.sarkar@intel.com
Subject: Re: [PATCH v14 bpf-next 00/18] mvneta: introduce XDP multi-buffer support
Date: Tue, 21 Sep 2021 12:03:56 +0200	[thread overview]
Message-ID: <2C4CB8CA-1234-4761-8F74-49A198F94880@redhat.com> (raw)
In-Reply-To: <87ilyu50kl.fsf@toke.dk>

On 21 Sep 2021, at 0:44, Toke Høiland-Jørgensen wrote:

> Jakub Kicinski <kuba@kernel.org> writes:
>
>> 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.
>
> Hmm, good question. On the one hand it seems like an easy optimisation
> to add, but on the other hand maybe the caller has other logic that can
> better know how/when to omit the check.
>
> Hmm, but the helper needs to check it anyway, doesn't it? At least it
> can't just blindly memcpy() if the source and destination would be the
> same...
>
>> 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).
>
> I guess we could play some trickery with stuffing offset/len/flags into
> one or two u64s to save an argument or two?
>
>> 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.
>
> Ah, didn't think of that; so then it really becomes a generic
> xdp_store_bytes()-type helper? Might be useful, I suppose. Adding
> headers is certainly a fairly common occurrence, but dunno to what
> extent they'd be copied wholesale from a map (hadn't thought about doing
> that before either).

Sorry for commenting late but I was busy and had to catch up on emails...

I like the idea, as these APIs are exactly what I proposed in April, https://lore.kernel.org/bpf/FD3E6E08-DE78-4FBA-96F6-646C93E88631@redhat.com/

I did not call it flush, as it can be used as a general function to copy data to a specific location.

//Eelco