linux-kernel - Re: [PATCH] virtio-net: put virtio net header inline with data

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Date:	Fri, 07 Jun 2013 11:42:43 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	"Michael S. Tsirkin" <mst@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Jason Wang <jasowang@...hat.com>,
	Cong Wang <amwang@...hat.com>, Amos Kong <akong@...hat.com>,
	Dave Jones <davej@...hat.com>,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Cc:	qemu-devel@...gnu.org
Subject: Re: [PATCH] virtio-net: put virtio net header inline with data

"Michael S. Tsirkin" <mst@...hat.com> writes:
> For small packets we can simplify xmit processing by linearizing buffers
> with the header: most packets seem to have enough head room we can use
> for this purpose.
>
> Since some older hypervisors (e.g. qemu before version 1.5)
> required that header is the first s/g element,
> we need a feature bit for this.

OK, we know this is horrible.  But I will sleep better knowing that we
this feature need never make it into a final 1.0 spec, since it can be
assumed at that point...

>  	pr_debug("%s: xmit %p %pM\n", vi->dev->name, skb, dest);
> +	if (vi->mergeable_rx_bufs)
> +		hdr_len = sizeof hdr->mhdr;
> +	else
> +		hdr_len = sizeof hdr->hdr;
> +
> +	can_push = vi->any_header_sg &&
> +		!((unsigned long)skb->data & (__alignof__(*hdr) - 1)) &&
> +		!skb_header_cloned(skb) && skb_headroom(skb) >= hdr_len;

Idle thought: how often does this fail?  Would it suck if we copied
headers which didn't let us prepend data?  Or could we bump
dev->hard_header_len appropriately?

Thanks,
Rusty.
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