Re: [PATCH 01/15] x86/split_lock: Add CONFIG and enumerate #AC exception for split locked access feature

[Dave Hansen]

On 05/15/2018 08:41 AM, Fenghua Yu wrote:
> On Tue, May 15, 2018 at 08:36:35AM -0700, Dave Hansen wrote:
>> On 05/14/2018 11:52 AM, Fenghua Yu wrote:
>>> +#include <linux/printk.h>
>>> +#include <asm/msr.h>
>>> +
>>> +static bool split_lock_ac_supported;
>> Was there a reason not to use an X86_FEATURE* bit for this?
> The feature is not enumerated in cpuid. It's detected by writing 1
> to bit 29 in MSR 0x33.
> 
> So it can't fit in X86_FEATURE, right?

We have X86_FEATURE_*'s for lots of things, even software constructs
that have no representation in the hardware CPUID.