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

[Jan Kiszka <jan.kiszka@siemens.com>]

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?

>>
>> I just want make L0's unrestricted guest mode support available for L1
>> (in fact, I already did this in my hacking branch).
>>
> Don't you need EPT for that too?

Sure. As I wrote, I'm using Nadav's patches of last August against some
3.6 baseline.

Jan

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