netdev - Re: [PATCH v3 bpf-next 2/2] net: xdp: introduce xdp_prepare

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Date:   Tue, 15 Dec 2020 21:36:46 +0100
From:   Lorenzo Bianconi <lorenzo@...nel.org>
To:     Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Cc:     Lorenzo Bianconi <lorenzo.bianconi@...hat.com>,
        Daniel Borkmann <daniel@...earbox.net>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        ast@...nel.org, brouer@...hat.com, alexander.duyck@...il.com,
        saeed@...nel.org
Subject: Re: [PATCH v3 bpf-next 2/2] net: xdp: introduce xdp_prepare_buff
 utility routine

> On Tue, Dec 15, 2020 at 04:06:20PM +0100, Lorenzo Bianconi wrote:
> > > On 12/15/20 2:47 PM, Lorenzo Bianconi wrote:
> > > [...]
> > > > > > diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
> > > > > > index 329397c60d84..61d3f5f8b7f3 100644
> > > > > > --- a/drivers/net/xen-netfront.c
> > > > > > +++ b/drivers/net/xen-netfront.c
> > > > > > @@ -866,10 +866,8 @@ static u32 xennet_run_xdp(struct netfront_queue *queue, struct page *pdata,
> > > > > >   	xdp_init_buff(xdp, XEN_PAGE_SIZE - XDP_PACKET_HEADROOM,
> > > > > >   		      &queue->xdp_rxq);
> > > > > > -	xdp->data_hard_start = page_address(pdata);
> > > > > > -	xdp->data = xdp->data_hard_start + XDP_PACKET_HEADROOM;
> > > > > > +	xdp_prepare_buff(xdp, page_address(pdata), XDP_PACKET_HEADROOM, len);
> > > > > >   	xdp_set_data_meta_invalid(xdp);
> > > > > > -	xdp->data_end = xdp->data + len;
> > > > > >   	act = bpf_prog_run_xdp(prog, xdp);
> > > > > >   	switch (act) {
> > > > > > diff --git a/include/net/xdp.h b/include/net/xdp.h
> > > > > > index 3fb3a9aa1b71..66d8a4b317a3 100644
> > > > > > --- a/include/net/xdp.h
> > > > > > +++ b/include/net/xdp.h
> > > > > > @@ -83,6 +83,18 @@ xdp_init_buff(struct xdp_buff *xdp, u32 frame_sz, struct xdp_rxq_info *rxq)
> > > > > >   	xdp->rxq = rxq;
> > > > > >   }
> > > > > > +static inline void
> > > 
> > > nit: maybe __always_inline
> > 
> > ack, I will add in v4
> > 
> > > 
> > > > > > +xdp_prepare_buff(struct xdp_buff *xdp, unsigned char *hard_start,
> > > > > > +		 int headroom, int data_len)
> > > > > > +{
> > > > > > +	unsigned char *data = hard_start + headroom;
> > > > > > +
> > > > > > +	xdp->data_hard_start = hard_start;
> > > > > > +	xdp->data = data;
> > > > > > +	xdp->data_end = data + data_len;
> > > > > > +	xdp->data_meta = data;
> > > > > > +}
> > > > > > +
> > > > > >   /* Reserve memory area at end-of data area.
> > > > > >    *
> > > 
> > > For the drivers with xdp_set_data_meta_invalid(), we're basically setting xdp->data_meta
> > > twice unless compiler is smart enough to optimize the first one away (did you double check?).
> > > Given this is supposed to be a cleanup, why not integrate this logic as well so the
> > > xdp_set_data_meta_invalid() doesn't get extra treatment?
> 
> That's what I was trying to say previously.
> 
> > 
> > we discussed it before, but I am fine to add it in v4. Something like:
> > 
> > static __always_inline void
> > xdp_prepare_buff(struct xdp_buff *xdp, unsigned char *hard_start,
> > 		 int headroom, int data_len, bool meta_valid)
> > {
> > 	unsigned char *data = hard_start + headroom;
> > 	
> > 	xdp->data_hard_start = hard_start;
> > 	xdp->data = data;
> > 	xdp->data_end = data + data_len;
> > 	xdp->data_meta = meta_valid ? data : data + 1;
> 
> This will introduce branch, so for intel drivers we're getting the
> overhead of one add and a branch. I'm still opting for a separate helper.
> 
> static __always_inline void
> xdp_prepare_buff(struct xdp_buff *xdp, unsigned char *hard_start,
> 		 int headroom, int data_len)
> {
> 	unsigned char *data = hard_start + headroom;
> 
> 	xdp->data_hard_start = hard_start;
> 	xdp->data = data;
> 	xdp->data_end = data + data_len;
> 	xdp_set_data_meta_invalid(xdp);
> }
> 
> static __always_inline void
> xdp_prepare_buff_meta(struct xdp_buff *xdp, unsigned char *hard_start,
> 		      int headroom, int data_len)
> {
> 	unsigned char *data = hard_start + headroom;
> 
> 	xdp->data_hard_start = hard_start;
> 	xdp->data = data;
> 	xdp->data_end = data + data_len;
> 	xdp->data_meta = data;
> }

yes, to follow-up the possible approaches we have here are:

- have 2 different helpers (xdp_prepare_buff_meta and xdp_prepare_buff) as
  suggested by Maciej
- move the data_meta initialization out of the helper and do it in each
  driver
- use the current approach and overwrite data_meta with
  xdp_set_data_meta_invalid() when necessary
- introduce a branch in order to have just one helper

what is the best for you?

Regards,
Lorenzo

> 
> > }
> > 
> > Regards,
> > Lorenzo
> > 
> > > 
> > > Thanks,
> > > Daniel
> > > 
> 
> 

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