netdev - Re: [kvm-devel] QEMU PIC indirection patch for in-kernel APIC work

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Date:	Mon, 09 Apr 2007 19:46:24 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Avi Kivity <avi@...ranet.com>
Cc:	Ingo Molnar <mingo@...e.hu>, kvm-devel@...ts.sourceforge.net,
	netdev <netdev@...r.kernel.org>
Subject: Re: [kvm-devel] QEMU PIC indirection patch for in-kernel APIC work

On Mon, 2007-04-09 at 10:10 +0300, Avi Kivity wrote:
> Rusty Russell wrote:
> > 	I'm a little puzzled by your response.  Hmm...
> >
> > 	lguest's userspace network frontend does exactly as many copies as
> > Ingo's in-host-kernel code.  One from the Guest, one to the Guest.
> 
> kvm pvnet is suboptimal now.  The number of copies could be reduced by 
> two (to zero), by constructing an skb that points to guest memory.  
> Right now, this can only be done in-kernel.

Sorry, you lost me here.  You mean both input and output copies can be
eliminated?  Or are you talking about another two copies somewhere?

But I don't get this "we can enhance the kernel but not userspace" vibe
8(

> With current userspace networking interfaces, one cannot build a network 
> device that has less than one copy on transmit, because sendmsg() *must* 
> copy the data (as there is no completion notification).

Why are you talking about sendmsg()?  Perhaps this is where we're
getting tangled up.

We're dealing with the tun/tap device here, not a socket.

>  sendfilev(), 
> even if it existed, cannot be used: it is copyless, but lacks completion 
> notification.  It is useful only on unchanging data like read-only files.

Again, sendfile is a *much* harder problem than sending a single packet
once, which is the question here.

Rusty.

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