From: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>
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@xxxxxxxxxxxxxxx <linux-hyperv-
>> owner@xxxxxxxxxxxxxxx> On Behalf Of David Miller
>> Sent: Tuesday, January 21, 2020 5:05 AM
>> To: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>
>> Cc: sashal@xxxxxxxxxx; linux-hyperv@xxxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx;
>> KY Srinivasan <kys@xxxxxxxxxxxxx>; Stephen Hemminger
>> <sthemmin@xxxxxxxxxxxxx>; olaf@xxxxxxxxx; vkuznets
>> <vkuznets@xxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx
>> Subject: Re: [PATCH V2,net-next, 1/2] hv_netvsc: Add XDP support
>>
>> From: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>
>> 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.
