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

[Yu, Yu-cheng]

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

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