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

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

On 2013-03-07 13:21, 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.

Jan

-- 
Siemens AG, Corporate Technology, CT RTC ITP SDP-DE
Corporate Competence Center Embedded Linux