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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 18 May 2011 14:56:02 +0300

On Wed, May 18, 2011 at 01:47:33PM +0200, Michał Mirosław wrote:
> W dniu 18 maja 2011 13:17 użytkownik Michael S. Tsirkin
> <[email-masked]> napisał:
> > On Wed, May 18, 2011 at 01:10:50PM +0200, Michał Mirosław wrote:
> >> 2011/5/18 Michael S. Tsirkin <[email-masked]>:
> >> > 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 <[email-masked]>:
> >> >> > > 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.
> > That's a fundamental property of zero copy transmit.
> > You can't let the application/guest reuse the memory until
> > no one looks at it anymore.
> 
> One more question: is userspace (or whatever is sending those packets)
> denied from modifying passed pages? I assume it is, but just want to
> be sure.
> 
> Best Regards,
> Michał Mirosław

Good point.

It's not denied in the sense that it still can modify them if it's
buggy (the pages might not be read-only).
But well-behaved userspace won't modify them until the callback
is invoked.

That would be a problem if the underlying device is
a bridge where we might try to e.g. filter these packets -
data can get modified after the filter. We'd have to copy
whatever the filter accesses and use the copy - it's rarely
the data itself.

That's not normally a problem for macvtap connected to a physical NIC,
as that already bypasses any and all filtering.

But that's another limitation we should note in the comment,
and another reason to limit to specific devices.

-- 
MST