Re: [PATCH 04/11] x86/acpi: Check MWAIT feature instead of CPUID level

Dave Hansen
2024-11-21T21:46:22Z

On 11/21/24 07:33, Sean Christopherson wrote:
> On Wed, Nov 20, 2024, Dave Hansen wrote:
>> From: Dave Hansen <dave.hansen@linux.intel.com>
>>
>> I think this code is possibly buggy. The CPU could have a high
>> c->cpuid_level and not support MWAIT at all.
>
> Reading CPUID.0x5 is totally fine in that case though. Wasteful and pointless,
> but functionally ok. If the CPU provides non-zero values when MWAIT is unsupported,
> then that's a broken CPU.

That's a good point.

I was mostly worried about consuming *bad* data from CPUID.5, but the
leaf check takes care of that now.

>> diff -puN arch/x86/kernel/acpi/cstate.c~mwait-leaf-checks-3 arch/x86/kernel/acpi/cstate.c
>> --- a/arch/x86/kernel/acpi/cstate.c~mwait-leaf-checks-3 2024-11-20 11:44:17.225650902 -0800
>> +++ b/arch/x86/kernel/acpi/cstate.c 2024-11-20 11:44:17.225650902 -0800
>> @@ -173,7 +173,7 @@ int acpi_processor_ffh_cstate_probe(unsi
>> struct cpuinfo_x86 *c = &cpu_data(cpu);
>> long retval;
>>
>> - if (!cpu_cstate_entry || c->cpuid_level < CPUID_MWAIT_LEAF)
>> + if (!cpu_cstate_entry || cpu_has(c, X86_FEATURE_MWAIT))
>
> Someone didn't test this :-)

Guilty.

I'll just drop this.