RE: [PATCH V2,net-next, 1/2] hv_netvsc: Add XDP support

[Haiyang Zhang <haiyangz@microsoft.com>]

> -----Original Message-----
> From: linux-kernel-owner@vger.kernel.org <linux-kernel-
> owner@vger.kernel.org> On Behalf Of David Miller
> Sent: Tuesday, January 21, 2020 4:28 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>; linux-kernel@vger.kernel.org
> Subject: Re: [PATCH V2,net-next, 1/2] hv_netvsc: Add XDP support
> 
> From: Haiyang Zhang <haiyangz@microsoft.com>
> Date: Tue, 21 Jan 2020 18:53:28 +0000
> 
> > Sorry I was replying too quickly. See more detailed explanation below.
> >
> >> -----Original Message-----
> >> From: linux-hyperv-owner@vger.kernel.org <linux-hyperv-
> >> owner@vger.kernel.org> On Behalf Of David Miller
> >> Sent: Tuesday, January 21, 2020 5:05 AM
> >> 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>; linux-kernel@vger.kernel.org
> >> Subject: Re: [PATCH V2,net-next, 1/2] hv_netvsc: Add XDP support
> >>
> >> From: Haiyang Zhang <haiyangz@microsoft.com>
> >> Date: Mon, 20 Jan 2020 14:22:36 -0800
> >>
> >> > +u32 netvsc_run_xdp(struct net_device *ndev, struct netvsc_channel
> *nvchan,
> >> > +		   struct xdp_buff *xdp)
> >> > +{
> >> > +	struct page *page = NULL;
> >> > +	void *data = nvchan->rsc.data[0];
> >> > +	u32 len = nvchan->rsc.len[0];
> >> > +	struct bpf_prog *prog;
> >> > +	u32 act = XDP_PASS;
> >>
> >> Please use reverse christmas tree ordering of local variables.
> > Will do.
> >
> >>
> >> > +	xdp->data_hard_start = page_address(page);
> >> > +	xdp->data = xdp->data_hard_start + NETVSC_XDP_HDRM;
> >> > +	xdp_set_data_meta_invalid(xdp);
> >> > +	xdp->data_end = xdp->data + len;
> >> > +	xdp->rxq = &nvchan->xdp_rxq;
> >> > +	xdp->handle = 0;
> >> > +
> >> > +	memcpy(xdp->data, data, len);
> >>
> >> Why can't the program run directly on nvchan->rsc.data[0]?
> >
> > The Azure/Hyper-V synthetic NIC receive buffer doesn't provide
> > headroom for XDP. We thought about re-use the RNDIS header space, but
> > it's too small. So we decided to copy the packets to a page buffer for
> > XDP. And, most of our VMs on Azure have Accelerated  Network (SRIOV)
> > enabled, so most of the packets run on VF NIC. The synthetic NIC is
> > considered as a fallback data-path. So the data copy on netvsc won't
> > impact performance significantly.
> 
> You need to explain this in your commit message otherwise every reviewer
> with XDP expertiece will ask the same question.

Will do.

Thanks,
- Haiyang