On 21 Sep 2021, at 0:44, Toke Høiland-Jørgensen wrote:
> Jakub Kicinski <kuba@xxxxxxxxxx> 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@xxxxxxxxxx/
I did not call it flush, as it can be used as a general function to copy data to a specific location.
//Eelco
