> Lorenzo Bianconi <lorenzo@xxxxxxxxxx> writes:
>
> >> On Thu, 4 Nov 2021 18:35:32 +0100 Lorenzo Bianconi wrote:
> >> > This change adds support for tail growing and shrinking for XDP multi-buff.
> >> >
> >> > When called on a multi-buffer packet with a grow request, it will always
> >> > work on the last fragment of the packet. So the maximum grow size is the
> >> > last fragments tailroom, i.e. no new buffer will be allocated.
> >> >
> >> > When shrinking, it will work from the last fragment, all the way down to
> >> > the base buffer depending on the shrinking size. It's important to mention
> >> > that once you shrink down the fragment(s) are freed, so you can not grow
> >> > again to the original size.
> >>
> >> > +static int bpf_xdp_mb_increase_tail(struct xdp_buff *xdp, int offset)
> >> > +{
> >> > + struct skb_shared_info *sinfo = xdp_get_shared_info_from_buff(xdp);
> >> > + skb_frag_t *frag = &sinfo->frags[sinfo->nr_frags - 1];
> >> > + int size, tailroom;
> >> > +
> >> > + tailroom = xdp->frame_sz - skb_frag_size(frag) - skb_frag_off(frag);
> >>
> >> I know I complained about this before but the assumption that we can
> >> use all the space up to xdp->frame_sz makes me uneasy.
> >>
> >> Drivers may not expect the idea that core may decide to extend the
> >> last frag.. I don't think the skb path would ever do this.
> >>
> >> How do you feel about any of these options:
> >> - dropping this part for now (return an error for increase)
> >> - making this an rxq flag or reading the "reserved frag size"
> >> from rxq (so that drivers explicitly opt-in)
> >> - adding a test that can be run on real NICs
> >> ?
> >
> > I think this has been added to be symmetric with bpf_xdp_adjust_tail().
> > I do think there is a real use-case for it so far so I am fine to just
> > support the shrink part.
> >
> > @Eelco, Jesper, Toke: any comments on it?
>
> Well, tail adjust is useful for things like encapsulations that need to
> add a trailer. Don't see why that wouldn't be something people would
> want to do for jumboframes as well?
>
I agree this would be useful for protocols that add a trailer.
> Not sure I get what the issue is with this either? But having a test
> that can be run to validate this on hardware would be great in any case,
> I suppose - we've been discussing more general "compliance tests" for
> XDP before...
what about option 2? We can add a frag_size field to rxq [0] that is set by
the driver initializing the xdp_buff. frag_size set to 0 means we can use
all the buffer.
Regards,
Lorenzo
[0] pahole -C xdp_rxq_info vmlinux
struct xdp_rxq_info {
struct net_device * dev; /* 0 8 */
u32 queue_index; /* 8 4 */
u32 reg_state; /* 12 4 */
struct xdp_mem_info mem; /* 16 8 */
unsigned int napi_id; /* 24 4 */
/* size: 64, cachelines: 1, members: 5 */
/* padding: 36 */
} __attribute__((__aligned__(64)));
>
> -Toke
>
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