ARM: KVM: Teach some form of type-safety to kvm_call_hyp

Just like on arm64, and for the same reasons, kvm_call_hyp removes
any form of type safety when calling into HYP. But we can still
try to tell the compiler what we're trying to achieve.

Here, we can add code that would do the function call if it wasn't
guarded by an always-false predicate. Hopefully, the compiler is
dumb enough to do the type checking and clever enough to not emit
the corresponding code...
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Christoffer Dall <christoffer.dall@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@arm.com>

arch/arm/include/asm/kvm_host.h

@@ -215,8 +215,33 @@ int kvm_arm_copy_reg_indices(struct kvm_vcpu *vcpu, u64 __user *indices);
 int kvm_arm_get_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg);
 int kvm_arm_set_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg);
 
-unsigned long kvm_call_hyp(void *hypfn, ...);
-#define kvm_call_hyp_ret(f, ...) kvm_call_hyp(f, ##__VA_ARGS__)
+unsigned long __kvm_call_hyp(void *hypfn, ...);
+
+/*
+ * The has_vhe() part doesn't get emitted, but is used for type-checking.
+ */
+#define kvm_call_hyp(f, ...)						\
+	do {								\
+		if (has_vhe()) {					\
+			f(__VA_ARGS__);					\
+		} else {						\
+			__kvm_call_hyp(kvm_ksym_ref(f), ##__VA_ARGS__); \
+		}							\
+	} while(0)
+
+#define kvm_call_hyp_ret(f, ...)					\
+	({								\
+		typeof(f(__VA_ARGS__)) ret;				\
+									\
+		if (has_vhe()) {					\
+			ret = f(__VA_ARGS__);				\
+		} else {						\
+			ret = __kvm_call_hyp(kvm_ksym_ref(f),		\
+					     ##__VA_ARGS__);		\
+		}							\
+									\
+		ret;							\
+	})
 
 void force_vm_exit(const cpumask_t *mask);
 int __kvm_arm_vcpu_get_events(struct kvm_vcpu *vcpu,
@@ -268,7 +293,7 @@ static inline void __cpu_init_hyp_mode(phys_addr_t pgd_ptr,
 	 * compliant with the PCS!).
 	 */
 
-	kvm_call_hyp((void*)hyp_stack_ptr, vector_ptr, pgd_ptr);
+	__kvm_call_hyp((void*)hyp_stack_ptr, vector_ptr, pgd_ptr);
 }
 
 static inline void __cpu_init_stage2(void)

arch/arm/kvm/hyp/hyp-entry.S

@@ -176,7 +176,7 @@ THUMB(	orr	lr, lr, #PSR_T_BIT	)
 	msr	spsr_cxsf, lr
 	ldr	lr, =panic
 	msr	ELR_hyp, lr
-	ldr	lr, =kvm_call_hyp
+	ldr	lr, =__kvm_call_hyp
 	clrex
 	eret
 ENDPROC(__hyp_do_panic)

arch/arm/kvm/interrupts.S

@@ -42,7 +42,7 @@
  *   r12:     caller save
  *   rest:    callee save
  */
-ENTRY(kvm_call_hyp)
+ENTRY(__kvm_call_hyp)
 	hvc	#0
 	bx	lr
-ENDPROC(kvm_call_hyp)
+ENDPROC(__kvm_call_hyp)