netdev - Re: [PATCH V5 2/6 net-next] netdevice.h: Add zero-copy flag in netdevice

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Date:	Wed, 18 May 2011 11:06:00 +0200
From:	Michał Mirosław <mirqus@...il.com>
To:	Shirley Ma <mashirle@...ibm.com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	Ben Hutchings <bhutchings@...arflare.com>,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Avi Kivity <avi@...hat.com>, Arnd Bergmann <arnd@...db.de>,
	netdev@...r.kernel.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V5 2/6 net-next] netdevice.h: Add zero-copy flag in netdevice

W dniu 18 maja 2011 01:44 użytkownik Shirley Ma <mashirle@...ibm.com> napisał:
> On Wed, 2011-05-18 at 00:58 +0200, Michał Mirosław wrote:
>> W dniu 18 maja 2011 00:28 użytkownik Shirley Ma <mashirle@...ibm.com>
>> napisał:
>> > On Tue, 2011-05-17 at 23:48 +0200, Michał Mirosław wrote:
>> >> 2011/5/17 Shirley Ma <mashirle@...ibm.com>:
>> >> > Looks like to use a new flag requires more time/work. I am
>> thinking
>> >> > whether we can just use HIGHDMA flag to enable zero-copy in
>> macvtap
>> >> to
>> >> > avoid the new flag for now since mavctap uses real NICs as lower
>> >> device?
>> >>
>> >> Is there any other restriction besides requiring driver to not
>> recycle
>> >> the skb? Are there any drivers that recycle TX skbs?
>> > Not more other restrictions, skb clone is OK. pskb_expand_head()
>> looks
>> > OK to me from code review.
>>
>> > Currently there is no drivers recycle TX skbs.
>>
>> So why do you require the target device to have some flags at all?
> We could use macvtap to check lower device HIGHDMA to enable zero-copy,
> but I am not sure whether it is sufficient. If it's sufficient then we
> don't need to use a new flag here. To be safe, it's better to use a new
> flag to enable each device who can pass zero-copy test.

>> Do I understand correctly, that this zero-copy feature is about
>> packets received from VMs?
> Yes, packets sent from VMs, and received in local host for TX zero-copy
> here.

What is the zero-copy test? On some arches the HIGHDMA is not needed
at all so might be not enabled on anything. It looks like the correct
test would be per-packet check of !illegal_highdma() or maybe
NETIF_F_SG as returned from harmonize_features(). For virtual devices
or other software forwarding this might lead to skb_linearize() in
some cases, but is it that bad?

Best Regards,
Michał Mirosław
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