x86/bugs: Move the X86_FEATURE_USE_IBPB check into callers

indirect_branch_prediction_barrier() only performs the MSR write if
X86_FEATURE_USE_IBPB is set, using alternative_msr_write(). In
preparation for removing X86_FEATURE_USE_IBPB, move the feature check
into the callers so that they can be addressed one-by-one, and use
X86_FEATURE_IBPB instead to guard the MSR write.

Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
Acked-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20250227012712.3193063-2-yosry.ahmed@linux.dev

Author: yosrym93

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

@@ -515,7 +515,7 @@ extern u64 x86_pred_cmd;
 
 static inline void indirect_branch_prediction_barrier(void)
 {
-	alternative_msr_write(MSR_IA32_PRED_CMD, x86_pred_cmd, X86_FEATURE_USE_IBPB);
+	alternative_msr_write(MSR_IA32_PRED_CMD, x86_pred_cmd, X86_FEATURE_IBPB);
 }
 
 /* The Intel SPEC CTRL MSR base value cache */

arch/x86/kernel/cpu/bugs.c

@@ -2272,7 +2272,7 @@ static int ib_prctl_set(struct task_struct *task, unsigned long ctrl)
 		if (ctrl == PR_SPEC_FORCE_DISABLE)
 			task_set_spec_ib_force_disable(task);
 		task_update_spec_tif(task);
-		if (task == current)
+		if (task == current && cpu_feature_enabled(X86_FEATURE_USE_IBPB))
 			indirect_branch_prediction_barrier();
 		break;
 	default:

arch/x86/kvm/svm/svm.c

@@ -1565,7 +1565,8 @@ static void svm_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 	if (sd->current_vmcb != svm->vmcb) {
 		sd->current_vmcb = svm->vmcb;
 
-		if (!cpu_feature_enabled(X86_FEATURE_IBPB_ON_VMEXIT))
+		if (!cpu_feature_enabled(X86_FEATURE_IBPB_ON_VMEXIT) &&
+		    cpu_feature_enabled(X86_FEATURE_USE_IBPB))
 			indirect_branch_prediction_barrier();
 	}
 	if (kvm_vcpu_apicv_active(vcpu))

arch/x86/kvm/vmx/nested.c

@@ -5026,7 +5026,8 @@ void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 vm_exit_reason,
 	 * doesn't isolate different VMCSs, i.e. in this case, doesn't provide
 	 * separate modes for L2 vs L1.
 	 */
-	if (guest_cpu_cap_has(vcpu, X86_FEATURE_SPEC_CTRL))
+	if (guest_cpu_cap_has(vcpu, X86_FEATURE_SPEC_CTRL) &&
+	    cpu_feature_enabled(X86_FEATURE_USE_IBPB))
 		indirect_branch_prediction_barrier();
 
 	/* Update any VMCS fields that might have changed while L2 ran */

arch/x86/kvm/vmx/vmx.c

@@ -1477,7 +1477,8 @@ void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu,
 		 * performs IBPB on nested VM-Exit (a single nested transition
 		 * may switch the active VMCS multiple times).
 		 */
-		if (!buddy || WARN_ON_ONCE(buddy->vmcs != prev))
+		if (cpu_feature_enabled(X86_FEATURE_USE_IBPB) &&
+		    (!buddy || WARN_ON_ONCE(buddy->vmcs != prev)))
 			indirect_branch_prediction_barrier();
 	}
 

arch/x86/mm/tlb.c

@@ -437,7 +437,8 @@ static void cond_mitigation(struct task_struct *next)
 		 * both have the IBPB bit set.
 		 */
 		if (next_mm != prev_mm &&
-		    (next_mm | prev_mm) & LAST_USER_MM_IBPB)
+		    (next_mm | prev_mm) & LAST_USER_MM_IBPB &&
+		    cpu_feature_enabled(X86_FEATURE_USE_IBPB))
 			indirect_branch_prediction_barrier();
 	}
 
@@ -447,8 +448,8 @@ static void cond_mitigation(struct task_struct *next)
 		 * different context than the user space task which ran
 		 * last on this CPU.
 		 */
-		if ((prev_mm & ~LAST_USER_MM_SPEC_MASK) !=
-					(unsigned long)next->mm)
+		if ((prev_mm & ~LAST_USER_MM_SPEC_MASK) != (unsigned long)next->mm &&
+		    cpu_feature_enabled(X86_FEATURE_USE_IBPB))
 			indirect_branch_prediction_barrier();
 	}