Messages in this thread | | | Subject | Re: [PATCH v15 03/26] x86/fpu/xstate: Introduce CET MSR XSAVES supervisor states | From | "Yu, Yu-cheng" <> | Date | Tue, 1 Dec 2020 14:35:18 -0800 |
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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
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