netdev - Re: [PATCH net-next 8/9] igc: Enable RX via AF

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Message-ID: <20210414173142.4dccbba1@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Wed, 14 Apr 2021 17:31:42 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     "Joseph, Jithu" <jithu.joseph@...el.com>
Cc:     "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "dvorax.fuxbrumer@...ux.intel.com" <dvorax.fuxbrumer@...ux.intel.com>,
        "Fijalkowski, Maciej" <maciej.fijalkowski@...el.com>,
        "sassmann@...hat.com" <sassmann@...hat.com>,
        "Gomes, Vinicius" <vinicius.gomes@...el.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "Neftin, Sasha" <sasha.neftin@...el.com>,
        "Lifshits, Vitaly" <vitaly.lifshits@...el.com>,
        "Nguyen, Anthony L" <anthony.l.nguyen@...el.com>,
        "Desouza, Ederson" <ederson.desouza@...el.com>,
        "bjorn.topel@...el.com" <bjorn.topel@...el.com>
Subject: Re: [PATCH net-next 8/9] igc: Enable RX via AF_XDP zero-copy

On Wed, 14 Apr 2021 23:59:22 +0000 Joseph, Jithu wrote:
> On Wed, 2021-04-14 at 16:25 -0700, Jakub Kicinski wrote:
> > On Wed, 14 Apr 2021 23:14:04 +0000 Joseph, Jithu wrote:  
> > > If h/w time-stamp is added by the NIC, then metasize will be non
> > > zero
> > > (as  xdp->data is advanced by the driver ) .  h/w ts  is still
> > > copied
> > > into "skb_hwtstamps(skb)->hwtstamp" by  the caller of this function
> > > igc_dispatch_skb_zc()  . Do you still want it to be copied into
> > > __skb_put(skb, ) area too ?   
> > 
> > If TS is prepended to the frame it should be saved (e.g. on the
> > stack)
> > before XDP program is called and gets the chance to overwrite it. The
> > metadata length when XDP program is called should be 0.  
> 
> When you say metadata length should be 0 above, Do you mean that when
> bpf_prog_run_xdp(prog, xdp) is invoked, xdp->data and xdp->data_meta
> should point to the same address ?

Correct, see xdp_prepare_buff().