Re: [PATCH v2] KVM: nVMX: Fix setting of CR0 and CR4 in guest mode

[Gleb Natapov <gleb@redhat.com>]

On Thu, Mar 07, 2013 at 01:48:55PM +0100, Jan Kiszka wrote:
> On 2013-03-07 13:21, Gleb Natapov wrote:
> > On Thu, Mar 07, 2013 at 01:18:24PM +0100, Jan Kiszka wrote:
> >> On 2013-03-07 13:05, Gleb Natapov wrote:
> >>> On Thu, Mar 07, 2013 at 12:57:27PM +0100, Jan Kiszka wrote:
> >>>>>> Ah, sorry, you are not seeing what I'm looking at: The test will change
> >>>>>> for L2 context once unrestricted guest mode is added. At that point, it
> >>>>>> makes more sense to split it into one version that checks against
> >>>>>> VMXON_CR0_ALWAYSON while in vmxon, targeting L1, and another that does
> >>>>>> more complex evaluation for L2, depending on nested_cpu_has2(vmcs12,
> >>>>>> SECONDARY_EXEC_UNRESTRICTED_GUEST).
> >>>>>>
> >>>>> Ah, OK. Hard to argue that those checks can be consolidated without
> >>>>> seeing them :) So you want to implement unrestricted L1 on restricted L0 and
> >>>>> let L0 emulate real mode of L2 directly?
> >>>>
> >>>> Err, no. :) Well, that emulation might even work but doesn't help unless
> >>>> you also emulate EPT (not unrestricted guest mode without EPT support -
> >>>> according to the spec).
> >>> Yes, of course EPT is needed, but patches are available :) I think it
> >>> should speedup L2 real mode substantially. No need to go to L1 for each
> >>> instruction emulation and L1 will have to exit to L0 many times during
> >>> emulation of some instructions.
> >>
> >> The point is: If you already have EPT on the host, you likely also have
> >> native unrestricted guest mode. You just need to expose it and adjust
> >> some minor things (like this bug here) along the way. Not sure how many
> >> CPUs had EPT but no unrestricted guest mode. Do you have numbers?
> >>
> > AFAIK every single one before Westmere. Nehalem does no have it for
> > sure.
> 
> OK. Hmm, will it be more than just faking unrestricted mode toward L1
> and emulating in L0 then (which should happen automagically)? Maybe I
> will play with this under unrestricted_guest=0 when I have some time.
> 
We will probably need to teach emulator about more instruction that may
cause vmexit.

--
			Gleb.