Re: [Xen-devel] [PATCH 0/14] Nested Virtualization: Overview

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 17/08/2010 11:33, "Christoph Egger" <Christoph.Egger@xxxxxxx> wrote:

>>> - implement HVM-on-HVM (instead of SVM-on-HVM)
>> 
>> Given that we don't have consensus on cross architecture nested
>> virtualization support, I am doubting why this is urgent for now.
> 
> The reason to be "urgent" is not the time. This is the best way from
> the software engineering side.
> 
>> I would prefer we make SVM-on-SVM and VMX-on-VMX work first. After that,
>> if you prove SVM-on-VMX has real performance gain (which I doubt), we can
>> see how to make a much generic effort to accomodate both natively nested
>> virtualization and cross architecture nested virtualization.
> 
> Tim and Keir made clear they don't want to have two implementations after
> I submitted my patch series the *first* time.

I think maybe this is an argument over two different things. To be clear we
want to support VMX-on-VMX and SVM-on-SVM. I assume this is what Christoph
means by HVM-on-HVM: any-like-on-like. In which case there is no
disagreement here.

Now, separately there is a debate to be had on how much code can be shared
in HVM-on-HVM, given the big differences between VMX and SVM. I would guess
that there will be at least things in the area of nested shadow and nested
HAP that can be shared, for example. Probably there is other stuff too. The
question is to what degree do we pursue that now rather than get divergent
stuff in tree and then go after it later. My mind isn't totally made up on
that; I don't know about Tim's.

 -- Keir



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