Subject: Re: [GIT PULL] AlacrityVM guest drivers for 2.6.33

From: Anthony Liguori <[email-masked]>
Date: Mon, 21 Dec 2009 10:37:03 -0600

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