Re: [Xen-users] Re: [Xen-devel] VMX Hardware

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 14 Aug 2005, at 13:23, Mogens Valentin wrote:

> I assume by VMX, you're talking about cpu/hardware with HW-support for 
> virtualisation, i.e. Intels Vanderbilt Technology (VT) and AMD' 
> Pacifica. If so, it's clear to me that AMD will be leading, because of 
> it's onchip memory controller, and hence reduced needs for software 
> support to do the page switching, for which Intel needs quite a lot of 
> software to emulate.
Not sure what you mean. Both technologies require the hypervisor to 
maintain shadow page tables. SVM (Pacifica) does have the advantage of 
a tagged TLB, so avoiding TLB flushes when switching between guest mode 
and hypervisor mode, and between guests, but we have no measurements so 
estimates of relative performance are pure conjecture.
> However, I'm not sufficiently familiar with Xen on this topic (just 
> getting started). What I've read is that Xen won't benefit from or 
> even use those HW-enabled mechanisms.
> Someone pls. correct me if I'm wrong here - I'd very much like to be 
> wrong. Would be a pity not to take advantage of such mechanisms, IMHO.
Xen will use VT and SVM to run unmodified operating systems. This will 
be useful for virtualising legacy system installations, and for 
supporting OSes where the source is not freely available.
Xen already has VT support, and I expect that SVM will also be 
supported by the end of the year. Hardware won't be generally available 
before then anyway.
 -- Keir


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