Re: [PATCH v5 1/9] x86/split_lock: Rework the initialization flow of split lock detection

[Thomas Gleixner]

Xiaoyao Li <xiaoyao.li@xxxxxxxxx> writes:
> On 3/24/2020 1:02 AM, Thomas Gleixner wrote:
>> Xiaoyao Li <xiaoyao.li@xxxxxxxxx> writes:
>> 
>>> Current initialization flow of split lock detection has following issues:
>>> 1. It assumes the initial value of MSR_TEST_CTRL.SPLIT_LOCK_DETECT to be
>>>     zero. However, it's possible that BIOS/firmware has set it.
>> 
>> Ok.
>> 
>>> 2. X86_FEATURE_SPLIT_LOCK_DETECT flag is unconditionally set even if
>>>     there is a virtualization flaw that FMS indicates the existence while
>>>     it's actually not supported.
>>>
>>> 3. Because of #2, KVM cannot rely on X86_FEATURE_SPLIT_LOCK_DETECT flag
>>>     to check verify if feature does exist, so cannot expose it to
>>>     guest.
>> 
>> Sorry this does not make anny sense. KVM is the hypervisor, so it better
>> can rely on the detect flag. Unless you talk about nested virt and a
>> broken L1 hypervisor.
>> 
>
> Yeah. It is for the nested virt on a broken L1 hypervisor.

Then please spell it out in the changelog. Changelogs which need crystalballs
to decode are pretty useless.

Thanks,

        tglx