Subject: Re: [PATCH v3 3/6] vbus: add a "vbus-proxy" bus model for vbus_driver objects

From: Avi Kivity <[email-masked]>
Date: Tue, 18 Aug 2009 10:38:39 +0300

On 08/18/2009 04:08 AM, Anthony Liguori wrote:
> I believe strongly that we should avoid putting things in the kernel 
> unless they absolutely have to be.  I'm definitely interested in 
> playing with vhost to see if there are ways to put even less in the 
> kernel.  In particular, I think it would be a big win to avoid 
> knowledge of slots in the kernel by doing ring translation in 
> userspace.  This implies a userspace transition in the fast path.  
> This may or may not be acceptable.  I think this is going to be a very 
> interesting experiment and will ultimately determine whether my 
> intuition about the cost of dropping to userspace is right or wrong.

I believe with a perfectly scaling qemu this should be feasible.  
Currently qemu is far from scaling perfectly, but inefficient userspace 
is not a reason to put things into the kernel.

Having a translated ring is also a nice solution for migration - 
userspace can mark the pages dirty while translating the receive ring.

Still, in-kernel translation is simple enough that I think we should 
keep it.

-- 
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