Re: Question regarding VMCS launch state

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On 09.04.2018 09:17, Jidong Xiao wrote:
> On Mon, Apr 9, 2018 at 1:10 AM, David Hildenbrand <david@xxxxxxxxxx> wrote:
>> On 09.04.2018 06:21, Jidong Xiao wrote:
>>> Hi,
>>>
>>> The Intel SDM says:
>>>
>>> "The launch state of a VMCS determines which VM-entry instruction
>>> should be used with that VMCS: the
>>> VMLAUNCH instruction requires a VMCS whose launch state is “clear”;
>>> the VMRESUME instruction requires a VMCS
>>> whose launch state is “launched”. A logical processor maintains a
>>> VMCS’s launch state in the corresponding VMCS
>>> region."
>>>
>>> "There are no other ways to modify the launch state of a VMCS (it
>>> cannot be modified using VMWRITE) and there
>>> is no direct way to discover it (it cannot be read using VMREAD).'
>>>
>>> According to the above description, the launch state is stored in the
>>> VMCS region, but it seems the SDM doesn't say where exactly the launch
>>> state is stored in the VMCS. For example, the VMCS data is organized
>>> into six logic groups: Guest-state area, Host-state area, VM-execution
>>> control fields, VM-exit control fields, VM-entry control fields,
>>> VM-exit information fields. However, I can't find the "launch state"
>>> in any of these fields.
>>>
>>> If it's in the VMCS region, then why can't I find it? and why it can't
>>> be discovered by VMREAD?
>>
>> Why do you assume the launch state has to be stored inside the VMCS region?
>>
>> It is just being stated that you cannot detect/modify it via
>> VMWRITE/VMREAD. Which is perfectly true if the launch state is stored
>> outside of the VMCS region.
>>
>> What the SDM states here simply is that the caller has to remember if a
>> VMCS has already been launched. This is what we do with the "launched"
>> variable.
>>
>> And this is also what we use for emulation of VMLAUNCH/VMRESUME in
>> nested_vmx_run().
>>
>>>
>>> I saw in the kvm source code a variable called launched is defined,
>>> which enables kvm to trace the launch state of the VMCS, but is this
>>> launch state really existing in the VMCS region? If so, where exactly
>>> is it?
>>
>> Nope, not in the region, it's glued to struct loaded_vmcs.
>>
> Oh, okay, thanks David, I see. The reason I assumed it's in the VMCS
> region is because the SDM says:
> 
> "A logical processor maintains a VMCS’s launch state in the
> corresponding VMCS region."
> 
> Since it's not really in the region, I guess the SDM is inaccurate on this.

Guess as the guest has to way to access it via VMREAD/VMWRITE it doesn't
make any difference, or do you have something in mind?

> 
>>
>> --
>>
>> Thanks,
>>
>> David / dhildenb


-- 

Thanks,

David / dhildenb



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