netdev - Re: [PATCH v2 net-next 7/9] bpf: helpers: add multibuffer support

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Message-ID: <20200904094759.GG2884@lore-desk>
Date:   Fri, 4 Sep 2020 11:47:59 +0200
From:   Lorenzo Bianconi <lorenzo@...nel.org>
To:     Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org, davem@...emloft.net,
        lorenzo.bianconi@...hat.com, brouer@...hat.com,
        echaudro@...hat.com, sameehj@...zon.com, kuba@...nel.org,
        john.fastabend@...il.com, daniel@...earbox.net, ast@...nel.org,
        shayagr@...zon.com
Subject: Re: [PATCH v2 net-next 7/9] bpf: helpers: add multibuffer support

> On Thu, Sep 03, 2020 at 10:58:51PM +0200, Lorenzo Bianconi wrote:
> > From: Sameeh Jubran <sameehj@...zon.com>
> > 
> > The implementation is based on this [0] draft by Jesper D. Brouer.
> > 
> > Provided two new helpers:
> > 
> > * bpf_xdp_get_frag_count()
> > * bpf_xdp_get_frags_total_size()
> > 
> > [0] xdp mb design - https://github.com/xdp-project/xdp-project/blob/master/areas/core/xdp-multi-buffer01-design.org
> > Signed-off-by: Sameeh Jubran <sameehj@...zon.com>
> > Signed-off-by: Lorenzo Bianconi <lorenzo@...nel.org>
> > ---
> >  include/uapi/linux/bpf.h       | 14 ++++++++++++
> >  net/core/filter.c              | 39 ++++++++++++++++++++++++++++++++++
> >  tools/include/uapi/linux/bpf.h | 14 ++++++++++++
> >  3 files changed, 67 insertions(+)
> > 
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index c4a6d245619c..53db75095306 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -3590,6 +3590,18 @@ union bpf_attr {
> >   *
> >   *	Return
> >   *		0 on success, or a negative error in case of failure.

[...]

> > +
> 
> I only quickly jumped through series and IMHO this helper should be
> rewritten/optimized in a way that we bail out as early as possible if
> !xdp->mb as the rest of the code on that condition will do nothing and i'm
> not sure if compiler would optimize it.
> 
> 
> 	struct skb_shared_info *sinfo;
> 	int nfrags, i;
> 	int size = 0;
> 
> 	if (!xdp->mb)
> 		return 0;
> 
> 	sinfo = xdp_get_shared_info_from_buff(xdp);
> 
> 	nfrags = min(sinfo->nr_frags, MAX_SKB_FRAGS);
> 
> 	for (i = 0; i < nfrags; i++)
> 		size += skb_frag_size(&sinfo->frags[i]);
> 
> 	return size;
> 
> Thoughts?

I agree.

Regards,
Lorenzo

> 
> 
> > +	return size;
> > +}
> > +
> > +const struct bpf_func_proto bpf_xdp_get_frags_total_size_proto = {
> > +	.func		= bpf_xdp_get_frags_total_size,
> > +	.gpl_only	= false,
> > +	.ret_type	= RET_INTEGER,
> > +	.arg1_type	= ARG_PTR_TO_CTX,
> > +};
> > +
> >  BPF_CALL_2(bpf_xdp_adjust_tail, struct xdp_buff *, xdp, int, offset)
> >  {
> >  	void *data_hard_end = xdp_data_hard_end(xdp); /* use xdp->frame_sz */
> > @@ -6889,6 +6924,10 @@ xdp_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
> >  		return &bpf_xdp_adjust_tail_proto;
> >  	case BPF_FUNC_xdp_adjust_mb_header:
> >  		return &bpf_xdp_adjust_mb_header_proto;
> > +	case BPF_FUNC_xdp_get_frag_count:
> > +		return &bpf_xdp_get_frag_count_proto;
> > +	case BPF_FUNC_xdp_get_frags_total_size:
> > +		return &bpf_xdp_get_frags_total_size_proto;
> >  	case BPF_FUNC_fib_lookup:
> >  		return &bpf_xdp_fib_lookup_proto;
> >  #ifdef CONFIG_INET
> > diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> > index 392d52a2ecef..dd4669096cbb 100644
> > --- a/tools/include/uapi/linux/bpf.h
> > +++ b/tools/include/uapi/linux/bpf.h
> > @@ -3591,6 +3591,18 @@ union bpf_attr {
> >   *
> >   *	Return
> >   *		0 on success, or a negative error in case of failure.
> > + *
> > + * int bpf_xdp_get_frag_count(struct xdp_buff *xdp_md)
> > + *	Description
> > + *		Get the total number of frags for a given packet.
> > + *	Return
> > + *		The number of frags
> > + *
> > + * int bpf_xdp_get_frags_total_size(struct xdp_buff *xdp_md)
> > + *	Description
> > + *		Get the total size of frags for a given packet.
> > + *	Return
> > + *		The total size of frags for a given packet.
> >   */
> >  #define __BPF_FUNC_MAPPER(FN)		\
> >  	FN(unspec),			\
> > @@ -3743,6 +3755,8 @@ union bpf_attr {
> >  	FN(d_path),			\
> >  	FN(copy_from_user),		\
> >  	FN(xdp_adjust_mb_header),	\
> > +	FN(xdp_get_frag_count),		\
> > +	FN(xdp_get_frags_total_size),	\
> >  	/* */
> >  
> >  /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> > -- 
> > 2.26.2
> > 

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