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Re: [Xen-devel] Re: Does Xen also plan to move the back-end driver to th

To: "Anthony Liguori" <aliguori@xxxxxxxxxx>
Subject: Re: [Xen-devel] Re: Does Xen also plan to move the back-end driver to the stub domain for HVM?
From: "Liang Yang" <multisyncfe991@xxxxxxxxxxx>
Date: Mon, 19 Mar 2007 12:21:14 -0700
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"QEMU has direct access to hardware", does this mean the QEMU device model does not need to communicate with the native device driver which is also sitting in dom0?

----- Original Message ----- From: "Anthony Liguori" <aliguori@xxxxxxxxxx>
To: "Liang Yang" <multisyncfe991@xxxxxxxxxxx>
Cc: <xen-devel@xxxxxxxxxxxxxxxxxxx>
Sent: Monday, March 19, 2007 11:20 AM
Subject: [Xen-devel] Re: Does Xen also plan to move the back-end driver to the stub domain for HVM?

Liang Yang wrote:

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.


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?



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