[RFC PATCH v1 10/15] KVM: VMX: Use WRMSRNS or its immediate form when available

["Xin Li (Intel)" <xin@xxxxxxxxx>]

Signed-off-by: Xin Li (Intel) <xin@xxxxxxxxx>
---
 arch/x86/include/asm/msr-index.h |  6 ++++++
 arch/x86/kvm/vmx/vmenter.S       | 28 ++++++++++++++++++++++++----
 2 files changed, 30 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index e6134ef2263d..04244c3ba374 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -1226,4 +1226,10 @@
                                                * a #GP
                                                */
 
+/* Instruction opcode for WRMSRNS supported in binutils >= 2.40 */
+#define ASM_WRMSRNS            _ASM_BYTES(0x0f,0x01,0xc6)
+
+/* Instruction opcode for the immediate form RDMSR/WRMSRNS */
+#define ASM_WRMSRNS_RAX                _ASM_BYTES(0xc4,0xe7,0x7a,0xf6,0xc0)
+
 #endif /* _ASM_X86_MSR_INDEX_H */
diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
index f6986dee6f8c..9fae43723c44 100644
--- a/arch/x86/kvm/vmx/vmenter.S
+++ b/arch/x86/kvm/vmx/vmenter.S
@@ -64,6 +64,29 @@
        RET
 .endm
 
+/*
+ * Write EAX to MSR_IA32_SPEC_CTRL.
+ *
+ * Choose the best WRMSR instruction based on availability.
+ *
+ * Replace with 'wrmsrns' and 'wrmsrns %rax, $MSR_IA32_SPEC_CTRL' once 
binutils support them.
+ */
+.macro WRITE_EAX_TO_MSR_IA32_SPEC_CTRL
+       ALTERNATIVE_2 __stringify(mov $MSR_IA32_SPEC_CTRL, %ecx;                
\
+                                 xor %edx, %edx;                               
\
+                                 mov %edi, %eax;                               
\
+                                 ds wrmsr),                                    
\
+                     __stringify(mov $MSR_IA32_SPEC_CTRL, %ecx;                
\
+                                 xor %edx, %edx;                               
\
+                                 mov %edi, %eax;                               
\
+                                 ASM_WRMSRNS),                                 
\
+                     X86_FEATURE_WRMSRNS,                                      
\
+                     __stringify(xor %_ASM_AX, %_ASM_AX;                       
\
+                                 mov %edi, %eax;                               
\
+                                 ASM_WRMSRNS_RAX; .long MSR_IA32_SPEC_CTRL),   
\
+                     X86_FEATURE_MSR_IMM
+.endm
+
 .section .noinstr.text, "ax"
 
 /**
@@ -123,10 +146,7 @@ SYM_FUNC_START(__vmx_vcpu_run)
        movl PER_CPU_VAR(x86_spec_ctrl_current), %esi
        cmp %edi, %esi
        je .Lspec_ctrl_done
-       mov $MSR_IA32_SPEC_CTRL, %ecx
-       xor %edx, %edx
-       mov %edi, %eax
-       wrmsr
+       WRITE_EAX_TO_MSR_IA32_SPEC_CTRL
 
 .Lspec_ctrl_done:
 
-- 
2.49.0