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

From: Rusty Russell
Date: Thu Jun 06 2013 - 23:03:44 EST

"Michael S. Tsirkin" <mst@xxxxxxxxxx> 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?

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