netdev - Re: [PATCH v2 net-next 2/9] xdp: initialize xdp

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Message-ID: <20200904075441.GD2884@lore-desk>
Date:   Fri, 4 Sep 2020 09:54:41 +0200
From:   Lorenzo Bianconi <lorenzo@...nel.org>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org, davem@...emloft.net,
        lorenzo.bianconi@...hat.com, echaudro@...hat.com,
        sameehj@...zon.com, kuba@...nel.org, john.fastabend@...il.com,
        daniel@...earbox.net, ast@...nel.org, shayagr@...zon.com,
        intel-wired-lan <intel-wired-lan@...ts.osuosl.org>
Subject: Re: [PATCH v2 net-next 2/9] xdp: initialize xdp_buff mb bit to 0 in
 all XDP drivers

> On Thu,  3 Sep 2020 22:58:46 +0200
> Lorenzo Bianconi <lorenzo@...nel.org> wrote:
> 
> > diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> > index 0b675c34ce49..20c8fd3cd4a3 100644
> > --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> > +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> > @@ -2298,6 +2298,7 @@ static int ixgbe_clean_rx_irq(struct ixgbe_q_vector *q_vector,
> >  #if (PAGE_SIZE < 8192)
> >  	xdp.frame_sz = ixgbe_rx_frame_truesize(rx_ring, 0);
> >  #endif
> > +	xdp.mb = 0;
> >  
> >  	while (likely(total_rx_packets < budget)) {
> >  		union ixgbe_adv_rx_desc *rx_desc;
> 
> In this ixgbe driver you are smart and init the xdp.mb bit outside the
> (like xdp.frame_sz, when frame_sz is constant).   This is a nice
> optimization, but the driver developer that adds XDP multi-buffer
> support must remember to reset it.  The patch itself is okay, it is
> just something to keep in-mind when reviewing/changing drivers.

yes, I have just decided to avoid unnecessary instructions for the moment.

Regards,
Lorenzo

> 
> -- 
> Best regards,
>   Jesper Dangaard Brouer
>   MSc.CS, Principal Kernel Engineer at Red Hat
>   LinkedIn: http://www.linkedin.com/in/brouer
> 

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