netdev - RE: [PATCHv9 net-next 2/4] sunvnet: make transmit path zero-copy in the kernel

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Message-ID: <063D6719AE5E284EB5DD2968C1650D6D174A2CD2@AcuExch.aculab.com>
Date:	Thu, 2 Oct 2014 09:06:44 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Raghuram Kothakota' <Raghuram.Kothakota@...cle.com>,
	David L Stevens <david.stevens@...cle.com>
CC:	David Miller <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Sowmini Varadhan <sowmini.varadhan@...cle.com>
Subject: RE: [PATCHv9 net-next 2/4] sunvnet: make transmit path zero-copy in
 the kernel

From: Raghuram Kothakota
> Sorry I am late in providing my comments, but I feel it is important
> to share this comment.

A comment on the original patch...

...
> > @@ -811,14 +930,27 @@ static int vnet_start_xmit(struct sk_buff *skb, struct net_device *dev)
> >
> > 	d = vio_dring_cur(dr);
> >
> > -	tx_buf = port->tx_bufs[dr->prod].buf;
> > -	skb_copy_from_linear_data(skb, tx_buf + VNET_PACKET_SKIP, skb->len);
> > +	txi = dr->prod;
> > +
> > +	freeskbs = vnet_clean_tx_ring(port, &pending);
> > +
> > +	BUG_ON(port->tx_bufs[txi].skb);
> >
> > 	len = skb->len;
> > -	if (len < ETH_ZLEN) {
> > +	if (len < ETH_ZLEN)
> > 		len = ETH_ZLEN;
> > -		memset(tx_buf+VNET_PACKET_SKIP+skb->len, 0, len - skb->len);
> > +

Aren't you transmitting 'random' bytes from the end of the data?
Plausibly they might not even be mapped.

Also, for short frames the copy could well be faster - especially on systems
with non-trivial iommu.
It is also worth checking whether the original copy was aligned.

	David




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