Re: [PATCH 1/2] x86/split_lock: Don't try to handle user split lock in TDX guest

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

On 11/26/2025 7:25 PM, Kiryl Shutsemau wrote:
> On Wed, Nov 26, 2025 at 06:02:03PM +0800, Xiaoyao Li wrote:
>> When the host enables split lock detection feature, the split lock from
>> guests (normal or TDX) triggers #AC. The #AC caused by split lock access
>> within a normal guest triggers a VM Exit and is handled in the host.
>> The #AC caused by split lock access within a TDX guest does not trigger
>> a VM Exit and instead it's delivered to the guest self.
>>
>> The default "warning" mode of handling split lock depends on being able
>> to temporarily disable detection to recover from the split lock event.
>> But the MSR that disables detection is not accessible to a guest.
>>
>> This means that TDX guests today can not disable the feature or use
>> the "warning" mode (which is the default). But, they can use the "fatal"
>> mode.
>>
>> Force TDX guests to use the "fatal" mode.
>>
>> Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
>> ---
>>   arch/x86/kernel/cpu/bus_lock.c | 17 ++++++++++++++++-
>>   1 file changed, 16 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kernel/cpu/bus_lock.c b/arch/x86/kernel/cpu/bus_lock.c
>> index 981f8b1f0792..f278e4ea3dd4 100644
>> --- a/arch/x86/kernel/cpu/bus_lock.c
>> +++ b/arch/x86/kernel/cpu/bus_lock.c
>> @@ -315,9 +315,24 @@ void bus_lock_init(void)
>>   	wrmsrq(MSR_IA32_DEBUGCTLMSR, val);
>>   }
>>   
>> +static bool split_lock_fatal(void)
>> +{
>> +	if (sld_state == sld_fatal)
>> +		return true;
>> +
>> +	/*
>> +	 * TDX guests can not disable split lock detection.
>> +	 * Force them into the fatal behavior.
>> +	 */
>> +	if (cpu_feature_enabled(X86_FEATURE_TDX_GUEST))
>> +		return true;
>> +
>> +	return false;
>> +}
>> +
>>   bool handle_user_split_lock(struct pt_regs *regs, long error_code)
>>   {
>> -	if ((regs->flags & X86_EFLAGS_AC) || sld_state == sld_fatal)
>> +	if ((regs->flags & X86_EFLAGS_AC) || split_lock_fatal())
>>   		return false;
> 
> Maybe it would be cleaner to make it conditional on
> cpu_model_supports_sld instead of special-casing TDX guest?
> 
> #AC on any platfrom when we didn't asked for it suppose to be fatal, no?

But TDX is the only one has such special non-architectural behavior.

For example, for normal VMs under KVM, the behavior is x86 
architectural. MSR_TEST_CTRL is not accessible to normal VMs, and no 
split lock #AC will be delivered to the normal VMs because it's handled 
by KVM.

>>   	split_lock_warn(regs->ip);
>>   	return true;
>> -- 
>> 2.43.0
>>
>