Subject: Re: [PATCH bpf-next 2/2] net: xdp: add xdp_update_skb_shared_info utility routine

From: Lorenzo Bianconi <[email-masked]>
Date: Mon, 12 Jul 2021 22:22:34 +0200

> Lorenzo Bianconi wrote:
> > Introduce xdp_update_skb_shared_info routine to update frags array
> > metadata from a given xdp_buffer/xdp_frame. We do not need to reset
> > frags array since it is already initialized by the driver.
> > Rely on xdp_update_skb_shared_info in mvneta driver.
> 
> Some more context here would really help. I had to jump into the mvneta
> driver to see what is happening.

Hi John,

ack, you are right. I will add more context next time. Sorry for the noise.

> 
> So as I read this we have a loop processing the descriptor in
> mvneta_rx_swbm()
> 
>  mvneta_rx_swbm()
>    while (rx_proc < budget && rx_proc < rx_todo) {
>      if (rx_status & MVNETA_RXD_FIRST_DESC) ...
>      else {
>        mvneta_swbm_add_rx_fragment()
>      }
>      ..
>      if (!rx_status & MVNETA_RXD_LAST_DESC)
>          continue;
>      ..
>      if (xdp_prog)
>        mvneta_run_xdp(...)
>    }
> 
> roughly looking like above. First question, do you ever hit
> !MVNETA_RXD_LAST_DESC today? I assume this is avoided by hardware
> setup when XDP is enabled, otherwise _run_xdp() would be
> broken correct? Next question, given last descriptor bit
> logic whats the condition to hit the code added in this patch?
> wouldn't we need more than 1 descriptor and then we would
> skip the xdp_run... sorry lost me and its probably easier
> to let you give the flow vs spending an hour trying to
> track it down.

I will point it out in the new commit log, but this is a preliminary patch for
xdp multi-buff support. In the current codebase xdp_update_skb_shared_info()
is run just when the NIC is not running in XDP mode (please note
mvneta_swbm_add_rx_fragment() is run even if xdp_prog is NULL).
When we add xdp multi-buff support, xdp_update_skb_shared_info() will run even
in XDP mode since we will remove the MTU constraint.

In the current codebsae the following condition can occur in non-XDP mode if
the packet is split on 3 or more descriptors (e.g. MTU 9000):

if (!(rx_status & MVNETA_RXD_LAST_DESC))
   continue;

> 
> But, in theory as you handle a hardware discriptor you can build
> up a set of pages using them to create a single skb rather than a
> skb per descriptor. But don't we know if pfmemalloc should be
> done while we are building the frag list? Can't se just set it
> vs this for loop in xdp_update_skb_shared_info(),

I added pfmemalloc code in xdp_update_skb_shared_info() in order to reuse it
for the xdp_redirect use-case (e.g. whenever we redirect a xdp multi-buff
in a veth or in a cpumap). I have a pending patch where I am using
xdp_update_skb_shared_info in __xdp_build_skb_from_frame().

> 
> > +	for (i = 0; i < nr_frags; i++) {
> > +		struct page *page = skb_frag_page(&sinfo->frags[i]);
> > +
> > +		page = compound_head(page);
> > +		if (page_is_pfmemalloc(page)) {
> > +			skb->pfmemalloc = true;
> > +			break;
> > +		}
> > +	}
> > +}
> 
> ...
> 
> > diff --git a/drivers/net/ethernet/marvell/mvneta.c b/drivers/net/ethernet/marvell/mvneta.c
> > index 361bc4fbe20b..abf2e50880e0 100644
> > --- a/drivers/net/ethernet/marvell/mvneta.c
> > +++ b/drivers/net/ethernet/marvell/mvneta.c
> > @@ -2294,18 +2294,29 @@ mvneta_swbm_add_rx_fragment(struct mvneta_port *pp,
> >  	rx_desc->buf_phys_addr = 0;
> >  
> >  	if (data_len > 0 && xdp_sinfo->nr_frags < MAX_SKB_FRAGS) {
> > -		skb_frag_t *frag = &xdp_sinfo->frags[xdp_sinfo->nr_frags++];
> > +		skb_frag_t *frag = &xdp_sinfo->frags[xdp_sinfo->nr_frags];
> >  
> >  		skb_frag_off_set(frag, pp->rx_offset_correction);
> >  		skb_frag_size_set(frag, data_len);
> >  		__skb_frag_set_page(frag, page);
> > +		/* We don't need to reset pp_recycle here. It's already set, so
> > +		 * just mark fragments for recycling.
> > +		 */
> > +		page_pool_store_mem_info(page, rxq->page_pool);
> > +
> > +		/* first fragment */
> > +		if (!xdp_sinfo->nr_frags)
> > +			xdp_sinfo->gso_type = *size;
> 
> Would be nice to also change 'int size' -> 'unsigned int size' so the
> types matched. Presumably you really can't have a negative size.
> 

ack

> Also how about giving gso_type a better name. xdp_sinfo->size maybe?

I did it in this way in order to avoid adding a union in skb_shared_info.
What about adding an inline helper to set/get it? e.g.

static inline u32 xdp_get_data_len(struct skb_shared_info *sinfo)
{
    return sinfo->gso_type;
}

static inline void xdp_set_data_len(struct skb_shared_info *sinfo, u32 datalen)
{
    sinfo->gso_type = datalen;
}

Regards,
Lorenzo

> 
> 
> > +		xdp_sinfo->nr_frags++;
> >  
> >  		/* last fragment */
> >  		if (len == *size) {
> >  			struct skb_shared_info *sinfo;
> >  
> >  			sinfo = xdp_get_shared_info_from_buff(xdp);
> > +			sinfo->xdp_frags_tsize = xdp_sinfo->nr_frags * PAGE_SIZE;
> >  			sinfo->nr_frags = xdp_sinfo->nr_frags;
> > +			sinfo->gso_type = xdp_sinfo->gso_type;
> >  			memcpy(sinfo->frags, xdp_sinfo->frags,
> >  			       sinfo->nr_frags * sizeof(skb_frag_t));
> >  		}
> 
> Thanks,
> John
>