On Tue, Sep 21, 2021 at 12:04 PM Eelco Chaudron <echaudro@xxxxxxxxxx> wrote:
>
>
>
> 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.
Here is some performance data (throughput) for this patch set on i40e
(40 Gbit/s NIC). All using the xdp_rxq_info sample and NO multi-buffer
packets.
With v14 only:
XDP_DROP: +4%
XDP_TX: +1%
XDP_PASS: -1%
With v14 plus multi-buffer support implemented in i40e courtesy of Tirtha:
XDP_DROP: +3%
XDP_TX: -1%
XDP_PASS: -2%
/Magnus
>
> //Eelco
>
