Subject: Re: [PATCH bpf-next v3 1/3] xsk: Support UMEM chunk_size > PAGE_SIZE

From: Maciej Fijalkowski <[email-masked]>
Date: Fri, 21 Apr 2023 11:37:37 +0200

On Tue, Apr 18, 2023 at 01:12:00PM +0200, Kal Cutter Conley wrote:

Hi there,

> > >> In addition, presumably when using this mode, the other XDP actions
> > >> (XDP_PASS, XDP_REDIRECT to other targets) would stop working unless we
> > >> add special handling for that in the kernel? We'll definitely need to
> > >> handle that somehow...
> > >
> > > I am not familiar with all the details here. Do you know a reason why
> > > these cases would stop working / why special handling would be needed?
> > > For example, if I have a UMEM that uses hugepages and XDP_PASS is
> > > returned, then the data is just copied into an SKB right? SKBs can
> > > also be created directly from hugepages AFAIK. So I don't understand
> > > what the issue would be. Can someone explain this concern?
> >
> > Well, I was asking :) It may well be that the SKB path just works; did
> > you test this? Pretty sure XDP_REDIRECT to another device won't, though?

for XDP_PASS we have to allocate a new buffer and copy the contents from
current xdp_buff that was backed by xsk_buff_pool and give the current one
back to pool. I am not sure if __napi_alloc_skb() is always capable of
handling len > PAGE_SIZE - i believe there might a particular combination
of settings that allows it, but if not we should have a fallback path that
would iterate over data and copy this to a certain (linear + frags) parts.
This implies non-zero effort that is needed for jumbo frames ZC support.

I can certainly test this out and play with it - maybe this just works, I
didn't check yet. Even if it does, then we need some kind of temporary
mechanism that will forbid loading ZC jumbo frames due to what Toke
brought up.

> >
> 
> I was also asking :-)
> 
> I tested that the SKB path is usable today with this patch.
> Specifically, sending and receiving large jumbo packets with AF_XDP
> and that a non-multi-buffer XDP program could access the whole packet.
> I have not specifically tested XDP_REDIRECT to another device or
> anything with ZC since that is not possible without driver support.
> 
> My feeling is, there wouldn't be non-trivial issues here since this
> patchset changes nothing except allowing the maximum chunk size to be
> larger. The driver either supports larger MTUs with XDP enabled or it
> doesn't. If it doesn't, the frames are dropped anyway. Also, chunk
> size mismatches between two XSKs (e.g. with XDP_REDIRECT) would be
> something supported or not supported irrespective of this patchset.

Here is the comparison between multi-buffer and jumbo frames that I did
for ZC ice driver. Configured MTU was 8192 as this is the frame size for
aligned mode when working with huge pages. I am presenting plain numbers
over here from xdpsock.

Mbuf, packet size = 8192 - XDP_PACKET_HEADROOM
885,705pps - rxdrop frame_size=4096
806,307pps - l2fwd frame_size=4096
877,989pps - rxdrop frame_size=2048
773,331pps - l2fwd frame_size=2048

Jumbo, packet size = 8192 - XDP_PACKET_HEADROOM
893,530pps - rxdrop frame_size=8192
841,860pps - l2fwd frame_size=8192

Kal might say that multi-buffer numbers are imaginary as these patches
were never shown to the public ;) but now that we have extensive test
suite I am fixing some last issues that stand out, so we are asking for
some more patience over here... overall i was expecting that they will be
much worse when compared to jumbo frames, but then again i believe this
implementation is not ideal and can be improved. Nevertheless, jumbo
frames support has its value.