Subject: Re: [PATCH bpf-next 01/21] xsk: prepare 'options' in xdp_desc for multi-buffer use

From: Maciej Fijalkowski <[email-masked]>
Date: Wed, 24 May 2023 12:27:30 +0200

On Wed, May 24, 2023 at 10:56:21AM +0200, Sarkar, Tirthendu wrote:
> > -----Original Message-----
> > From: Alexei Starovoitov <[email-masked]>
> > Sent: Friday, May 19, 2023 10:44 PM
> > To: Stanislav Fomichev <[email-masked]>
> > Cc: Fijalkowski, Maciej <[email-masked]>; bpf
> > <[email-masked]>; Alexei Starovoitov <[email-masked]>; Daniel
> > Borkmann <[email-masked]>; Andrii Nakryiko <[email-masked]>;
> > Network Development <[email-masked]>; Karlsson, Magnus
> > <[email-masked]>; Sarkar, Tirthendu
> > <[email-masked]>; Björn Töpel <[email-masked]>
> > Subject: Re: [PATCH bpf-next 01/21] xsk: prepare 'options' in xdp_desc for
> > multi-buffer use
> > 
> > On Thu, May 18, 2023 at 12:22 PM Stanislav Fomichev <[email-masked]>
> > wrote:
> > >
> > > On 05/18, Maciej Fijalkowski wrote:
> > > > From: Tirthendu Sarkar <[email-masked]>
> > > >
> > > > Use the 'options' field in xdp_desc as a packet continuity marker. Since
> > > > 'options' field was unused till now and was expected to be set to 0, the
> > > > 'eop' descriptor will have it set to 0, while the non-eop descriptors
> > > > will have to set it to 1. This ensures legacy applications continue to
> > > > work without needing any change for single-buffer packets.
> > > >
> > > > Add helper functions and extend xskq_prod_reserve_desc() to use the
> > > > 'options' field.
> > > >
> > > > Signed-off-by: Tirthendu Sarkar <[email-masked]>
> > > > ---
> > > >  include/uapi/linux/if_xdp.h | 16 ++++++++++++++++
> > > >  net/xdp/xsk.c               |  8 ++++----
> > > >  net/xdp/xsk_queue.h         | 12 +++++++++---
> > > >  3 files changed, 29 insertions(+), 7 deletions(-)
> > > >
> > > > diff --git a/include/uapi/linux/if_xdp.h b/include/uapi/linux/if_xdp.h
> > > > index a78a8096f4ce..4acc3a9430f3 100644
> > > > --- a/include/uapi/linux/if_xdp.h
> > > > +++ b/include/uapi/linux/if_xdp.h
> > > > @@ -108,4 +108,20 @@ struct xdp_desc {
> > > >
> > > >  /* UMEM descriptor is __u64 */
> > > >
> > > > +/* Flag indicating that the packet continues with the buffer pointed out
> > by the
> > > > + * next frame in the ring. The end of the packet is signalled by setting
> > this
> > > > + * bit to zero. For single buffer packets, every descriptor has 'options'
> > set
> > > > + * to 0 and this maintains backward compatibility.
> > > > + */
> > > > +#define XDP_PKT_CONTD (1 << 0)
> > > > +
> > > > +/* Maximum number of descriptors supported as frags for a packet. So
> > the total
> > > > + * number of descriptors supported for a packet is
> > XSK_DESC_MAX_FRAGS + 1. The
> > > > + * max frags supported by skb is 16 for page sizes greater than 4K and 17
> > or
> > >
> > > This is now a config option CONFIG_MAX_SKB_FRAGS. Can we use it
> > > directly?
> > 
> > Also it doesn't look right to expose kernel internal config in uapi
> > especially since XSK_DESC_MAX_FRAGS is not guaranteed to be 16.
> 
> Ok, we have couple of options here:
> 
> Option 1:  We will define XSK_DESC_MAX_FRAGS to 17 now. This will ensure AF_XDP
>  applications will work on any system without any change since the MAX_SKB_FRAGS
>  is guaranteed to be at least 17.
> 
> Option 2: Instead of defining a new macro, we say max frags supported is same as
>  MAX_SKB_FRAGS as configured in your system. So use 17 or less frags if you want 
>  your app to work everywhere but you can go larger if you control the system.
> 
> Any suggestions ?
> 
> Also Alexei could you please clarify what you meant by ".. since XSK_DESC_MAX_FRAGS
>  is not guaranteed to be 16." ?

Maybe it would be better to put this define onto patch 08 so people would
see how it is used and get a feeling of it? Although it has a description
nothing says about it in commit message.

FWIW i'm voting for option 2, but also Alexei's comment is a bit unclear
to me, would be nice to hear more about it.