x86/bugs: KVM: Move VM_CLEAR_CPU_BUFFERS into SVM as SVM_CLEAR_CPU_BUFFERS

Now that VMX encodes its own sequence for clearing CPU buffers, move
VM_CLEAR_CPU_BUFFERS into SVM to minimize the chances of KVM botching a
mitigation in the future, e.g. using VM_CLEAR_CPU_BUFFERS instead of
checking multiple mitigation flags.

No functional change intended.

Reviewed-by: Brendan Jackman <jackmanb@google.com>
Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://patch.msgid.link/20251113233746.1703361-7-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/include/asm/nospec-branch.h

@@ -329,9 +329,6 @@
 #define CLEAR_CPU_BUFFERS \
 	ALTERNATIVE "", __CLEAR_CPU_BUFFERS, X86_FEATURE_CLEAR_CPU_BUF
 
-#define VM_CLEAR_CPU_BUFFERS \
-	ALTERNATIVE "", __CLEAR_CPU_BUFFERS, X86_FEATURE_CLEAR_CPU_BUF_VM
-
 #ifdef CONFIG_X86_64
 .macro CLEAR_BRANCH_HISTORY
 	ALTERNATIVE "", "call clear_bhb_loop", X86_FEATURE_CLEAR_BHB_LOOP

arch/x86/kvm/svm/vmenter.S

@@ -92,6 +92,8 @@
 	jmp 901b
 .endm
 
+#define SVM_CLEAR_CPU_BUFFERS \
+	ALTERNATIVE "", __CLEAR_CPU_BUFFERS, X86_FEATURE_CLEAR_CPU_BUF_VM
 
 /**
  * __svm_vcpu_run - Run a vCPU via a transition to SVM guest mode
@@ -170,7 +172,7 @@ SYM_FUNC_START(__svm_vcpu_run)
 	mov VCPU_RDI(%_ASM_DI), %_ASM_DI
 
 	/* Clobbers EFLAGS.ZF */
-	VM_CLEAR_CPU_BUFFERS
+	SVM_CLEAR_CPU_BUFFERS
 
 	/* Enter guest mode */
 3:	vmrun %_ASM_AX
@@ -339,7 +341,7 @@ SYM_FUNC_START(__svm_sev_es_vcpu_run)
 	mov KVM_VMCB_pa(%rax), %rax
 
 	/* Clobbers EFLAGS.ZF */
-	VM_CLEAR_CPU_BUFFERS
+	SVM_CLEAR_CPU_BUFFERS
 
 	/* Enter guest mode */
 1:	vmrun %rax