Re: [PATCH 4/5] kvm: qemu: Use vringfd to eliminate copies

[Rusty Russell <rusty@rustcorp.com.au>]

On Tuesday 17 June 2008 00:02:55 Anthony Liguori wrote:
> There's nothing that prevents zero-copy to be implemented for tun
> without vringfd.  In fact, I seem to recall that your earlier patches
> implemented zero-copy :-)

They didn't actually work.  You need to block until the data isn't being used 
any more (thread pool anyone?), or implement an aio interface.

> I like the vringfd model and I think it's a good way to move forward.
> My concern is that it introduces an extra syscall in the TX path.  Right
> now, we do a single write call whereas with vringfd we need to insert
> the TX packet into the queue, do a notify, and then wait for indication
> that the TX has succeeded.

If the guest wants notification of xmit, yes you need another syscall for 
that.  But it often doesn't (note: current vring tun ignored the NO_NOTIFY 
flag, but one thing at a time).

> I know we'll win with TSO but we don't need vringfd for TSO.  The jury's
> still out IMHO as to whether we should do vringfd or just try to merge
> TSO tun patches.

Note that we can do TSO in userspace, too.  No syscall reduction, but an VM 
exit reduction.

Cheers,
Rusty.