[PATCH v3 1/6] x86/cpufeature: add cpu feature bit for LKGS

Xin Li
2022-10-13T20:23:53Z

From: "H. Peter Anvin (Intel)" <hpa@zytor.com>

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

LKGS instruction is introduced with Intel FRED (flexible return and
event delivery) specificaton
https://cdrdv2.intel.com/v1/dl/getContent/678938.

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.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Xin Li <xin3.li@intel.com>
Link: https://lkml.org/lkml/2022/10/11/1139
---
arch/x86/include/asm/cpufeatures.h | 1 +
tools/arch/x86/include/asm/cpufeatures.h | 1 +
2 files changed, 2 insertions(+)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index b71f4f2ecdd5..3dc1a48c2796 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -308,6 +308,7 @@
/* Intel-defined CPU features, CPUID level 0x00000007:1 (EAX), word 12 */
#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_LKGS (12*32+ 18) /* "" Load "kernel" (userspace) gs */

/* AMD-defined CPU features, CPUID level 0x80000008 (EBX), word 13 */
#define X86_FEATURE_CLZERO (13*32+ 0) /* CLZERO instruction */
diff --git a/tools/arch/x86/include/asm/cpufeatures.h b/tools/arch/x86/include/asm/cpufeatures.h
index ef4775c6db01..9d45071d1730 100644
--- a/tools/arch/x86/include/asm/cpufeatures.h
+++ b/tools/arch/x86/include/asm/cpufeatures.h
@@ -308,6 +308,7 @@
/* Intel-defined CPU features, CPUID level 0x00000007:1 (EAX), word 12 */
#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_LKGS (12*32+ 18) /* "" Load "kernel" (userspace) gs */

/* AMD-defined CPU features, CPUID level 0x80000008 (EBX), word 13 */
#define X86_FEATURE_CLZERO (13*32+ 0) /* CLZERO instruction */
--
2.34.1