Re: [PATCH v9 bpf-next 10/14] bpf: add multi-buffer support to xdp copy helpers

From: Eelco Chaudron <echaudro@redhat.com>
To: John Fastabend <john.fastabend@gmail.com>
Cc: Lorenzo Bianconi <lorenzo@kernel.org>,
	bpf@vger.kernel.org, netdev@vger.kernel.org,
	lorenzo.bianconi@redhat.com, davem@davemloft.net,
	kuba@kernel.org, ast@kernel.org, daniel@iogearbox.net,
	shayagr@amazon.com, sameehj@amazon.com, dsahern@kernel.org,
	brouer@redhat.com, jasowang@redhat.com,
	alexander.duyck@gmail.com, saeed@kernel.org,
	maciej.fijalkowski@intel.com, magnus.karlsson@intel.com,
	tirthendu.sarkar@intel.com
Subject: Re: [PATCH v9 bpf-next 10/14] bpf: add multi-buffer support to xdp copy helpers
Date: Fri, 25 Jun 2021 10:25:26 +0200	[thread overview]
Message-ID: <911124FA-EE96-495C-AFC3-578B2D38247E@redhat.com> (raw)
In-Reply-To: <60d49690a87ae_2e84a2082c@john-XPS-13-9370.notmuch>

On 24 Jun 2021, at 16:28, John Fastabend wrote:

> Eelco Chaudron wrote:
>>
>>
>> On 23 Jun 2021, at 1:49, John Fastabend wrote:
>>
>>> Lorenzo Bianconi wrote:
>>>> From: Eelco Chaudron <echaudro@redhat.com>
>>>>
>>>> This patch adds support for multi-buffer for the following helpers:
>>>>   - bpf_xdp_output()
>>>>   - bpf_perf_event_output()
>>>>
>>>> Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
>>>> Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
>>>> ---
>>>
>>> Ah ok so at least xdp_output will work with all bytes. But this is
>>> getting close to having access into the frags so I think doing
>>> the last bit shouldn't be too hard?
>>
>>
>> Guess you are talking about multi-buffer access in the XDP program?
>>
>> I did suggest an API a while back, https://lore.kernel.org/bpf/FD3E6E08-DE78-4FBA-96F6-646C93E88631@redhat.com/ but I had/have not time to work on it. Guess the difficult part is to convince the verifier to allow the data to be accessed.
>
> Ah great I think we had the same idea I called it xdp_pull_data()
> though.
>
> Whats the complication though it looks like it can be done by simply
> moving the data and data_end pointers around then marking them
> invalidated. This way the verifier knows the program needs to
> rewrite them. I can probably look more into next week.
>
> From my first glance it looks relatively straight forward to do
> now. I really would like to avoid yet another iteration of
> programs features I have to discover and somehow work around
> if we can get the helper into this series. If you really don't
> have time I can probably take a look early next week on an
> RFC for something like above helper.

I’m on a small side project for the next 2 to 3 weeks, and after that, I have some PTO, so if you have time for an RFC, that will speed up this patchset.

Thanks,

Eelco

> .John