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

On 12/1/2020 2:26 PM, Dave Hansen wrote:

On 11/30/20 3:16 PM, Yu, Yu-cheng wrote:

quoted

quoted

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.

After more thoughts, I think it is better to just add X86_FEATURE_CET 
and not more.  We cannot predict what is going to happen later.
So, like what you suggested, X86_FEATURE_CET means (X86_FEATURE_SHSTK | 
X86_FEATURE_IBT).

Thanks,
Yu-cheng