Re: IA32_FEATURE_CONTROL MSR in nested virt

[Gleb Natapov <gleb@redhat.com>]

On Wed, Jul 03, 2013 at 10:48:33AM -0400, Bandan Das wrote:
> Gleb Natapov <gleb@redhat.com> writes:
> 
> > On Wed, Jul 03, 2013 at 04:24:33PM +0800, Arthur Chunqi Li wrote:
> >> Hi Gleb and Paolo,
> >> When I write test cases for nested virt and found that reading/writing
> >>  IA32_FEATURE_CONTROL will be simply ignored or return 0 (in
> >> arch/x86/kvm/vmx.c) in VM. Checking this MSR will be done by some
> >> hypervisors (e.g. NOVA) and may cause error then, so it is necessary
> >> to behave right when read/write it in VM.
> >> 
> > NOVA cannot write to it and expect anything but #GP since BIOSes usually
> > lock the MSR. So I agree with Paolo about returning 5 on read and #GP on
> > write. Later we can implement locking functionality and make BIOS
> > enable/disable nested vmx.
> 
> I vaguely recalled Nadav had posted a patch related to this MSR a
> while back and found this :
> 
> http://permalink.gmane.org/gmane.comp.emulators.kvm.devel/88478
> 
> Shouldn't this work ?
> 
It should, but it needs additional Seabios patch to enabled vmx. Also
the patch is incorrectly resets MSR on SIPI. There is no point in lock
bit if it can be reset by SIPI.

> >> Are there any difficulties to handle this MSR? I have two solutions.
> >> The first one is return the value of physical CPU's and always return
> >> true when write. This is simple but may behave as if it is a VM
> >> because write to it after VMXON will not return GP exception. This
> >> solution can solve most basic problems since this MSR is not commonly
> >> used. Another solution is adding a field in VCPU to handle this MSR.
> >> This is a complex but better method.
> >> 
> >> I think I can complete this if needed.
> >> 
> >> Thanks,
> >> Arthur
> >> 
> >> --
> >> Arthur Chunqi Li
> >> Department of Computer Science
> >> School of EECS
> >> Peking University
> >> Beijing, China
> >
> > --
> > 			Gleb.
> > --
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--
			Gleb.