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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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