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SubjectRe: [PATCH v14 03/26] x86/fpu/xstate: Introduce CET MSR XSAVES supervisor states
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On 10/12/2020 12:58 PM, Cyrill Gorcunov wrote:
> On Mon, Oct 12, 2020 at 08:38:27AM -0700, Yu-cheng Yu wrote:
> ...
>> /*
>> * x86-64 Task Priority Register, CR8
>> diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
>> index 038e19c0019e..705fd9b94e31 100644
>> --- a/arch/x86/kernel/fpu/xstate.c
>> +++ b/arch/x86/kernel/fpu/xstate.c
>> @@ -38,6 +38,9 @@ static const char *xfeature_names[] =
>> "Processor Trace (unused)" ,
>> "Protection Keys User registers",
>> "unknown xstate feature" ,
>> + "Control-flow User registers" ,
>> + "Control-flow Kernel registers" ,
>> + "unknown xstate feature" ,
>> };
>>
>> static short xsave_cpuid_features[] __initdata = {
>> @@ -51,6 +54,9 @@ static short xsave_cpuid_features[] __initdata = {
>> X86_FEATURE_AVX512F,
>> X86_FEATURE_INTEL_PT,
>> X86_FEATURE_PKU,
>> + -1, /* Unused */
>> + X86_FEATURE_SHSTK, /* XFEATURE_CET_USER */
>> + X86_FEATURE_SHSTK, /* XFEATURE_CET_KERNEL */
>> };
>
> Why do you need "-1" here in the array? The only 1:1 mapping is between
> the names itselves and values, not indices of arrays so i don't understand
> why we need this unused value. Sorry if it is a dumb questions and
> been discussed already.
>

The indices are used indirectly in fpu__init_system_xstate() to set bits
in xfeatures_mask_all, i.e.

xfeatures_mask_all &= ~BIT_ULL(i).

So they need to match the xstate feature bits.

Yu-cheng

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