netdev - Re: [PATCH v14 bpf-next 03/18] net: mvneta: update mb bit before passing the xdp buffer to eBPF layer

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Date:   Mon, 20 Sep 2021 11:45:20 +0300
From:   Shay Agroskin <shayagr@...zon.com>
To:     Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
CC:     Lorenzo Bianconi <lorenzo@...nel.org>, <bpf@...r.kernel.org>,
        <netdev@...r.kernel.org>, <davem@...emloft.net>, <kuba@...nel.org>,
        <ast@...nel.org>, <daniel@...earbox.net>,
        <john.fastabend@...il.com>, <dsahern@...nel.org>,
        <brouer@...hat.com>, <echaudro@...hat.com>, <jasowang@...hat.com>,
        <alexander.duyck@...il.com>, <saeed@...nel.org>,
        <maciej.fijalkowski@...el.com>, <magnus.karlsson@...el.com>,
        <tirthendu.sarkar@...el.com>, <toke@...hat.com>
Subject: Re: [PATCH v14 bpf-next 03/18] net: mvneta: update mb bit before passing the
 xdp buffer to eBPF layer


Lorenzo Bianconi <lorenzo.bianconi@...hat.com> writes:

>> 
>> Lorenzo Bianconi <lorenzo@...nel.org> writes:
>> 
>> > ...
>> > diff --git a/drivers/net/ethernet/marvell/mvneta.c
>> > b/drivers/net/ethernet/marvell/mvneta.c
>> > index 9d460a270601..0c7b84ca6efc 100644
>> > --- a/drivers/net/ethernet/marvell/mvneta.c
>> > +++ b/drivers/net/ethernet/marvell/mvneta.c
>> > ...
>> > @@ -2320,8 +2325,12 @@ mvneta_swbm_build_skb(struct 
>> > mvneta_port *pp,
>> > struct page_pool *pool,
>> >  		      struct xdp_buff *xdp, u32 desc_status)
>> >  {
>> >  	struct skb_shared_info *sinfo = 
>> >  xdp_get_shared_info_from_buff(xdp);
>> > -	int i, num_frags = sinfo->nr_frags;
>> >  	struct sk_buff *skb;
>> > +	u8 num_frags;
>> > +	int i;
>> > +
>> > +	if (unlikely(xdp_buff_is_mb(xdp)))
>> > +		num_frags = sinfo->nr_frags;
>> 
>> Hi,
>> nit, it seems that the num_frags assignment can be moved after 
>> the other
>> 'if' condition you added (right before the 'for' for 
>> num_frags), or even be
>> eliminated completely so that sinfo->nr_frags is used directly.
>> Either way it looks like you can remove one 'if'.
>> 
>> Shay
>
> Hi Shay,
>
> we can't move nr_frags assignement after build_skb() since this 
> field will be
> overwritten by that call.
>
> Regards,
> Lorenzo
>

Sorry, silly mistake of me.

Guess this assignment can be done anyway since there doesn't seem 
to be new cache misses introduced by it.
Anyway, nice catch, sorry for misleading you

>> 
>> >  	skb = build_skb(xdp->data_hard_start, PAGE_SIZE);
>> >  	if (!skb)
>> > @@ -2333,6 +2342,9 @@ mvneta_swbm_build_skb(struct 
>> > mvneta_port *pp,
>> > struct page_pool *pool,
>> >  	skb_put(skb, xdp->data_end - xdp->data);
>> >  	skb->ip_summed = mvneta_rx_csum(pp, desc_status);
>> > +	if (likely(!xdp_buff_is_mb(xdp)))
>> > +		goto out;
>> > +
>> >  	for (i = 0; i < num_frags; i++) {
>> >  		skb_frag_t *frag = &sinfo->frags[i];
>> >   @@ -2341,6 +2353,7 @@ mvneta_swbm_build_skb(struct 
>> >   mvneta_port *pp,
>> > struct page_pool *pool,
>> >  				skb_frag_size(frag), PAGE_SIZE);
>> >  	}
>> > +out:
>> >  	return skb;
>> >  }
>>