Re: [PATCH v2 1/3] KVM: nVMX: Don't advertise single context invalidation for invept

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

On 2014-04-11 20:35, Bandan Das wrote:
> Jan Kiszka <jan.kiszka@siemens.com> writes:
> 
>> On 2014-04-11 19:26, Bandan Das wrote:
>>> Jan Kiszka <jan.kiszka@siemens.com> writes:
>>>
>>>> On 2014-04-11 02:27, Bandan Das wrote:
>>>>> Marcelo Tosatti <mtosatti@redhat.com> writes:
>>>>>
>>>>>> On Mon, Mar 31, 2014 at 05:00:23PM -0400, Bandan Das wrote:
>>>>>>> For single context invalidation, we fall through to global
>>>>>>> invalidation in handle_invept() except for one case - when
>>>>>>> the operand supplied by L1 is different from what we have in
>>>>>>> vmcs12. However, typically hypervisors will only call invept
>>>>>>> for the currently loaded eptp, so the condition will
>>>>>>> never be true.
>>>>>>>
>>>>>>> Signed-off-by: Bandan Das <bsd@redhat.com>
>>>>>>
>>>>>> Bandan,
>>>>>>
>>>>>> Why not fix INVEPT single-context rather than removing it entirely?
>>>>>>
>>>>>> "Single-context. If the INVEPT type is 1, the logical processor
>>>>>> invalidates all guest-physical mappings and combined mappings associated
>>>>>> with the EP4TA specified in the INVEPT descriptor. Combined mappings for
>>>>>> that EP4TA are invalidated for all VPIDs and all PCIDs. (The instruction
>>>>>> may invalidate mappings associated with other EP4TAs.)"
>>>>>>
>>>>>> So just removing the "if (EPTP != CURRENT.EPTP) BREAK" should be enough.
>>>>>
>>>>> The single context invalidation in handle_invept() doesn't do 
>>>>> anything different. It just falls down to the global case.
>>>>> And the invept code in Xen and KVM both seemed to fall back
>>>>> to global invalidation if support for single context wasn't found.
>>>>> So, it was proposed not to advertise it at all.
>>>>>
>>>>> But rethinking this again, I agree with you. If there's a hypervisor
>>>>> with a  single context invept implmentation that does not fallback,
>>>>> this will unfortunately not work. Jan, do you agree with this ?
>>>>
>>>> A hypervisor that doesn't properly check the HW caps is just broken. And
>>>> one that mandates single context invalidation support is silly.
>>>
>>> Well, but we could make life a little bit easier for the unfortunate user
>>> using the broken hypervisor :) And advertising single context inavalidation
>>> doesn't really seem to have any downsides.
>>
>> Ok, let's try it this way: single-context invalidation is inherently
>> tied to VPID support (that's how you address a context). However, KVM
>> does not expose VPID to its guest. So this discussion is mood: no
>> hypervisor will make use of this feature as it has no means to fill in
>> the required parameter.
> 
> I thought (from the spec) invept single context invalidation
> takes the EP4TA as the second argument. invvpid single context
> however takes the VPID as its descriptor.

Oops, invept/invvpid mess-up while re-reading the spec - sorry.

> 
> The Xen L1 hypervisor was actually calling single context invept
> multiple times. That's how I hit this bug.

...and it's no longer doing it now, I suppose. The question remains,
which hypervisor we want to cater with a
"single-context-that-is-current-context" invalidation (that is my
understanding of Marcelo's proposal). On the other hand, if some
hypervisor actually uses invept to invalidate a non-current mapping, we
would regress compared to not exposing single context invept. Hope I got
this conclusion right. ;)

Jan

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