Re: [PATCH v2] x86/split_lock: Handle unexpected split lock as fatal

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 1/7/2026 11:19 PM, Dave Hansen wrote:
> On 1/7/26 05:49, Xiaoyao Li wrote:
>> +	/*
>> +	 * If #AC occurs on split lock without X86_FEATURE_SPLIT_LOCK_DETECT
>> +	 * the kernel cannot handle it by disabling the detection. Treat it as
>> +	 * fatal regardless of the sld_state.
>> +	 */
>> +	if (!cpu_feature_enabled(X86_FEATURE_SPLIT_LOCK_DETECT))
>> +		return true;
> 
> If #AC occurs on split lock without X86_FEATURE_SPLIT_LOCK_DETECT, that
> sounds more like a naughty hypervisor or buggy CPU that deserves a
> BUG_ON() rather than a situation where the kernel wants to move merrily
> along.

Yes. Such behavior is non-architectural.
1) If it happens on bare metal, the CPU is broken.
2) If it happens in guest, the hypervisor does something wrong.

> This also needs an explanation in the changelog about _why_
> X86_FEATURE_SPLIT_LOCK_DETECT isn't set and can't be set. It needs to
> explain why enumeration is not present *AND* is impossible to add.

The only case I know, where such non-architectural behavior can happen 
is TDX guest. It's a virtualization case and 
X86_FEATURE_SPLIT_LOCK_DETECT cannot be virtualized normally in a sane 
manner because MSR_TEST_CTRL is a per-core scope MSR. Enumerating 
X86_FEATURE_SPLIT_LOCK_DETECT to a guest means the guest is able to 
enable/disable the feature freely by its own. However, on the HT system, 
if the guest disables the feature for its vcpu, it will also disable the 
feature for the sibling CPU on the same core, where the host processes 
or other VMs might run. Even on non-HT system, allowing the guest to 
disable the feature will violate the host purpose of not getting any 
split lock when host sets to fatal mode.

On the other hand, the question can be "why getting #AC on the split 
lock if the feature is not available? and if it can be fixed to not get 
#AC?" For this question,

1) if it happens on bare metal, the CPU is broken. The kernel cannot fix it.

2) if it happens in guest, it should be the hypervisor enables the 
feature in hardward MSR when the guest is running. To fix it, the 
hypervisor can intercept the #AC and handle it itself instead of letting 
the #AC be delivered to the guest. This is what KVM already does for 
normal guests. However, for TDX guest, KVM cannot intercept #AC. It 
needs changes in TDX module to provide such ability.