Re: KVM & VMX root mode

Sander van Leeuwen
2007-09-01T17:24:00Z

Avi,

I'd like to make clear that we are talking about the situation where kvm
is *not* being used by any application.

It's my understanding that in some (many?) distributions kvm is actually
active on a permanent basis. I can't say whether this was done
by default or explicitly by the user. There is however no need to
activate vmx root mode when there is no client program that is using
the kvm extension.

From your comments I understand that you enable vmx root mode when the
kvm module is loaded. Regardless of whether an application
actually wishes to use the kvm extension.

Avi Kivity wrote:
> Sander van Leeuwen wrote:
>> Hi Avi,
>>
>> Our non-vmx mode fails, because the cpu is in vmx root mode.
>>
>> Two products that use vt-x for virtualization could perfectly
>> co-exist if both comply with the way Intel recommends people to use
>> vt-x.
>> See figure 19.1 in chapter 19.4 of the 'Intel 64 and IA-32
>> Architectures Software Developer's Manual'. VirtualBox is programmed to
>> follow these rules and therefor allows any other virtualizer to run
>> side-by-side.
>
> As far as I understand, kvm follows these rules. It enables vmx when
> loaded and disables then when unloaded.
>
>>
>> Currently KVM prevents us from using our generic virtualization
>> engine and does not allow anybody else to use the vt-x extensions
>> (without
>> explicitely leaving vmx root mode).
>
> Well, obviously kvm can't operate if you disable cr4.vmxe and/or
> switch paging off. The two solutions are not run-time compatible. I
> don't see why this is a problem as you should simply not run the
> product you aren't using, and everything should just work.
>
>>
>> As your product is included in the mainline Linux kernel and enabled
>> by default, it would be nice if you could follow Intel's
>> recommendations.
>
> kvm isn't enabled by default. It requires explicit user action to
> enter vmx mode ('modprobe kvm-intel').
>
>

--
Kind regards / mit freundlichen Gruessen / Met vriendelijke groet
Sander van Leeuwen

innoTek GmbH
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Germany