netdev - Re: [PATCH V2,net-next, 1/2] hv

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Date:   Tue, 21 Jan 2020 11:04:54 +0100 (CET)
From:   David Miller <davem@...emloft.net>
To:     haiyangz@...rosoft.com
Cc:     sashal@...nel.org, linux-hyperv@...r.kernel.org,
        netdev@...r.kernel.org, kys@...rosoft.com, sthemmin@...rosoft.com,
        olaf@...fle.de, vkuznets@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2,net-next, 1/2] hv_netvsc: Add XDP support

From: Haiyang Zhang <haiyangz@...rosoft.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.

> +	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]?

This data copy defeats the whole performance gain of using XDP.