On 10/12/2020 12:58 PM, Cyrill Gorcunov wrote:

On Mon, Oct 12, 2020 at 08:38:27AM -0700, Yu-cheng Yu wrote:
...

quoted

  /*
   * 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