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

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Jan Kiszka <jan.kiszka@xxxxxxxxxxx> writes:

> On 2014-04-11 02:27, Bandan Das wrote:
>> Marcelo Tosatti <mtosatti@xxxxxxxxxx> 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@xxxxxxxxxx>
>>>
>>> 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.

> Jan
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