netdev - Re: [GIT PULL] AlacrityVM guest drivers for 2.6.33

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Date:	Mon, 21 Dec 2009 10:37:03 -0600
From:	Anthony Liguori <anthony@...emonkey.ws>
To:	Gregory Haskins <gregory.haskins@...il.com>
CC:	Avi Kivity <avi@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	kvm@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
	torvalds@...ux-foundation.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	netdev@...r.kernel.org,
	"alacrityvm-devel@...ts.sourceforge.net" 
	<alacrityvm-devel@...ts.sourceforge.net>
Subject: Re: [GIT PULL] AlacrityVM guest drivers for 2.6.33

On 12/21/2009 10:04 AM, Gregory Haskins wrote:
> No, B and C definitely are, but A is lacking.  And the performance
> suffers as a result in my testing (vhost-net still throws a ton of exits
> as its limited by virtio-pci and only adds about 1Gb/s to virtio-u, far
> behind venet even with things like zero-copy turned off).
>    

How does virtio-pci limit vhost-net?  The only time exits should occur 
are when the guest notifies the host that something has been placed on 
the ring.  Since vhost-net has no tx mitigation scheme right now, the 
result may be that it's taking an io exit on every single packet but 
this is orthogonal to virtio-pci.

Since virtio-pci supports MSI-X, there should be no IO exits on 
host->guest notification other than EOI in the virtual APIC.  This is a 
light weight exit today and will likely disappear entirely with newer 
hardware.

Regards,

Anthony Liguori
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