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

[Xiaoyao Li]

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.