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Date:   Tue, 29 Oct 2019 14:59:05 -0700
From:   Stephen Hemminger <stephen@networkplumber.org>
To:     Haiyang Zhang <haiyangz@microsoft.com>
Cc:     Jakub Kicinski <jakub.kicinski@netronome.com>,
        "sashal@kernel.org" <sashal@kernel.org>,
        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        KY Srinivasan <kys@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        "olaf@aepfle.de" <olaf@aepfle.de>, vkuznets <vkuznets@redhat.com>,
        "davem@davemloft.net" <davem@davemloft.net>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH net-next, 3/4] hv_netvsc: Add XDP support
Message-ID: <20191029145905.414f86c3@hermes.lan>
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On Tue, 29 Oct 2019 19:17:25 +0000
Haiyang Zhang <haiyangz@microsoft.com> wrote:

> > -----Original Message-----
> > From: Jakub Kicinski <jakub.kicinski@netronome.com>
> > Sent: Monday, October 28, 2019 5:33 PM
> > To: Haiyang Zhang <haiyangz@microsoft.com>
> > Cc: sashal@kernel.org; linux-hyperv@vger.kernel.org;
> > netdev@vger.kernel.org; KY Srinivasan <kys@microsoft.com>; Stephen
> > Hemminger <sthemmin@microsoft.com>; olaf@aepfle.de; vkuznets
> > <vkuznets@redhat.com>; davem@davemloft.net; linux-
> > kernel@vger.kernel.org
> > Subject: Re: [PATCH net-next, 3/4] hv_netvsc: Add XDP support
> > 
> > On Mon, 28 Oct 2019 21:07:04 +0000, Haiyang Zhang wrote:  
> > > This patch adds support of XDP in native mode for hv_netvsc driver, and
> > > transparently sets the XDP program on the associated VF NIC as well.
> > >
> > > XDP program cannot run with LRO (RSC) enabled, so you need to disable  
> > LRO  
> > > before running XDP:
> > >         ethtool -K eth0 lro off
> > >
> > > XDP actions not yet supported:
> > >         XDP_TX, XDP_REDIRECT  
> > 
> > I don't think we want to merge support without at least XDP_TX these
> > days..  
> Thanks for your detailed comments --
> I'm working on the XDP_TX...
> 
> > 
> > And without the ability to prepend headers this may be the least
> > complete initial XDP implementation we've seen :(  
> The RNDIS packet buffer received by netvsc doesn't have a head room, but I'm
> considering copy the packets to the page buffer, with a head room space 
> reserved for XDP.


There is a small amount of headroom available by reusing the RNDIS
header and packet space. Looks like 40 bytes or so.