Re: Does Xen also plan to move the back-end driver to the stub domain for HVM?

[Anthony Liguori <aliguori@us.ibm.com>]

Liang Yang wrote:
> Hi,
> 
> Based on the roadmap on Xen summit, there is a plan to move QEMU and let 
> it run on the stub domain to improve HVM performance.

Using a stub domain won't improve HVM performance.  It will improve 
accountability and scalability but running a single HVM guest shouldn't 
see any improvement.

> However, comparing 
> with QEMU device model, it will be much easier to move BE driver and let 
> it run in stub domain instead of dom0 as BE part is running on the 
> kernel space (QEMU is running on user space).

Actually, this cannot make performance better since you're technically 
adding another layer of indirection in the picture.  Within dom0, 
qemu-dm has direct access to the hardware.  Fortunately, the Xen BE/FE 
model is quite good performance wise so there shouldn't be a performance 
regression here.

> but I'm little bit confused about the relationship between stub domain 
> and guest domain. Is the stub domain part of guest domain? Does each 
> guest domain have a stub domain which is created when the guest domain 
> is created?

A lot of this is still being worked out.  From a user perspective, the 
idea would be that creating an HVM domain would be identical to how it's 
done today.  What happens under the covers though remains to be seen.

Regards,

Anthony Liguori

> If the stub domain is part of guest domain, does porting device model to 
> stub domain compromise the orginial design purpose of isoloated devide 
> domain?
> 
> Thanks,
> 
> Liang