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

From: Anthony Liguori <[email-masked]>
Date: Tue, 22 Dec 2009 10:27:33 -0600

On 12/22/2009 10:21 AM, Andi Kleen wrote:
>> So far, the only actual technical advantage I've seen is that vbus avoids
>> EOI exits.
>>      
> The technical advantage is that it's significantly faster today.
>    

There are two separate pieces of code in question.  There are front-end 
drivers and there are back-end drivers.

Right now, there are only front-end drivers in the kernel today.  The 
combination of vbus front-end drivers and *kernel* back-end drivers are 
faster than the *combination* of virtio front-end drivers and 
*userspace* back-end drivers.

vhost-net is our kernel back-end driver.  No one has yet established 
that the combination of virtio front-end driver and kernel back-end 
driver is really significantly slower than vbus.

> Maybe your proposed alternative is as fast, or maybe it's not. Who knows?
>
>    
>> We think we understand why vbus does better than the current userspace
>> virtio backend.  That's why we're building vhost-net.  It's not done yet,
>> but our expectation is that it will do just as well if not better.
>>      
> That's the vapourware vs working code disconnect I mentioned. One side has hard
> numbers&working code and the other has expectations. I usually find it sad when the
> vapourware holds up the working code.
>    

We're not talking about vaporware.  vhost-net exists.

Regards,

Anthony Liguori

> -Andi