Re: [PATCH v10 4/6] x86/split_lock: Enumerate split lock detection if the IA32_CORE_CAPABILITIES MSR is not supported

[Andy Lutomirski]

> On Nov 21, 2019, at 4:25 PM, Fenghua Yu <fenghua.yu@xxxxxxxxx> wrote:
> 
> ïOn Thu, Nov 21, 2019 at 02:07:38PM -0800, Andy Lutomirski wrote:
>> 
>> 
>>>> On Nov 20, 2019, at 5:45 PM, Fenghua Yu <fenghua.yu@xxxxxxxxx> wrote:
>>> 
>>> ïArchitecturally the split lock detection feature is enumerated by
>>> IA32_CORE_CAPABILITIES MSR and future CPU models will indicate presence
>>> of the feature by setting bit 5. But the feature is present in a few
>>> older models where split lock detection is enumerated by the CPU models.
>>> 
>>> Use a "x86_cpu_id" table to list the older CPU models with the feature.
>>> 
>> 
>> This may need to be disabled if the HYPERVISOR bit is set.
> 
> How about just keeping this patch set as basic enabling code and
> keep HYPERVISOR out of scope as of now? KVM folks will have better
> handling of split lock in KVM once this patch set is available in
> the kernel.
> 
> 

You seem to be assuming that certain model CPUs have this feature even if not enumerated. You need to make sure you donât try to use it in a VM without the hypervisor giving you an indication that itâs available and permitted. My suggestion is to disable model-based enumeration if HYPERVISOR is set.  You should also consider probing the MSR to double check even if you donât think you have a hypervisor.