Re: KVM & VMX root mode

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

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').


-- 
Any sufficiently difficult bug is indistinguishable from a feature.


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