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

From: Michał Mirosław
Date: Wed May 18 2011 - 07:11:15 EST

2011/5/18 Michael S. Tsirkin <mst@xxxxxxxxxx>:
> On Tue, May 17, 2011 at 03:28:38PM -0700, Shirley Ma wrote:
>> On Tue, 2011-05-17 at 23:48 +0200, Michał Mirosław wrote:
>> > 2011/5/17 Shirley Ma <mashirle@xxxxxxxxxx>:
>> > > Hello Michael,
>> > >
>> > > 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 just recycling skbs, keeping reference to any of the pages in the
> skb. Another requirement is to invoke the callback
> in a timely fashion.  For example virtio-net doesn't limit the time until
> that happens (skbs are only freed when some other packet is
> transmitted), so we need to avoid zcopy for such (nested-virt)
> scenarious, right?

Hmm. But every hardware driver supporting SG will keep reference to
the pages until the packet is sent (or DMA'd to the device). This can
take a long time if hardware queue happens to stall for some reason.

Is it that you mean keeping a reference after all skbs pointing to the
pages are released?

>> Not more other restrictions, skb clone is OK. pskb_expand_head() looks
>> OK to me from code review.
> Hmm. pskb_expand_head calls skb_release_data while keeping
> references to pages. How is that ok? What do I miss?

It's making copy of the skb_shinfo earlier, so the pages refcount
stays the same.

Best Regards,
Michał Mirosław
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