x86/cpufeature: Add the CPU feature bit for LKGS

Add the CPU feature bit for LKGS (Load "Kernel" GS).

LKGS instruction is introduced with Intel FRED (flexible return and
event delivery) specification. Search for the latest FRED spec in most
search engines with this search pattern:

  site:intel.com FRED (flexible return and event delivery) specification

LKGS behaves like the MOV to GS instruction except that it loads
the base address into the IA32_KERNEL_GS_BASE MSR instead of the
GS segment’s descriptor cache, which is exactly what Linux kernel
does to load a user level GS base.  Thus, with LKGS, there is no
need to SWAPGS away from the kernel GS base.

[ mingo: Minor tweaks to the description. ]

Signed-off-by: H. Peter Anvin (Intel) <[email protected]>
Signed-off-by: Xin Li <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: H. Peter Anvin (Intel)

arch/x86/include/asm/cpufeatures.h

@@ -312,6 +312,7 @@
 #define X86_FEATURE_AVX_VNNI		(12*32+ 4) /* AVX VNNI instructions */
 #define X86_FEATURE_AVX512_BF16		(12*32+ 5) /* AVX512 BFLOAT16 instructions */
 #define X86_FEATURE_CMPCCXADD           (12*32+ 7) /* "" CMPccXADD instructions */
+#define X86_FEATURE_LKGS		(12*32+18) /* "" Load "kernel" (userspace) GS */
 #define X86_FEATURE_AMX_FP16		(12*32+21) /* "" AMX fp16 Support */
 #define X86_FEATURE_AVX_IFMA            (12*32+23) /* "" Support for VPMADD52[H,L]UQ */
 

tools/arch/x86/include/asm/cpufeatures.h

@@ -312,6 +312,7 @@
 #define X86_FEATURE_AVX_VNNI		(12*32+ 4) /* AVX VNNI instructions */
 #define X86_FEATURE_AVX512_BF16		(12*32+ 5) /* AVX512 BFLOAT16 instructions */
 #define X86_FEATURE_CMPCCXADD           (12*32+ 7) /* "" CMPccXADD instructions */
+#define X86_FEATURE_LKGS		(12*32+18) /* "" Load "kernel" (userspace) GS */
 #define X86_FEATURE_AMX_FP16		(12*32+21) /* "" AMX fp16 Support */
 #define X86_FEATURE_AVX_IFMA            (12*32+23) /* "" Support for VPMADD52[H,L]UQ */