Re: [PATCH v15 03/26] x86/fpu/xstate: Introduce CET MSR XSAVES supervisor states

Dave Hansen
2020-12-01T22:27:14Z

On 11/30/20 3:16 PM, Yu, Yu-cheng wrote:
>>
>> Do we have any other spots in the kernel where we care about:
>>
>>     boot_cpu_has(X86_FEATURE_SHSTK) ||
>>     boot_cpu_has(X86_FEATURE_IBT)
>>
>> ?  If so, we could also address this by declaring a software-defined
>> X86_FEATURE_CET and then setting it if SHSTK||IBT is supported, then we
>> just put that one feature in xsave_cpuid_features[].
>>
>
> These features have different CPUIDs but are complementary parts.  I
> don't know if someday there will be shadow-stack-only CPUs, but an
> IBT-only CPU is weird.  What if the kernel checks that the CPU has both
> features and presents only one feature flag (X86_FEATURE_CET), no
> X86_FEATURE_SHSTK or X86_FEATURE_IBT?

Logically, that's probably fine. But, X86_FEATURE_IBT/SHSTK are in a
non-scattered leaf, so we'll kinda define them whether we like it or
not. We'd have to go out of our way to *not* define them.