diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index 4a555f32885a8c..5e19e6e4c2ce0b 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -2022,7 +2022,7 @@ static void kvm_hv_hypercall_set_result(struct kvm_vcpu *vcpu, u64 result)
 {
 	bool longmode;
 
-	longmode = is_64_bit_mode(vcpu);
+	longmode = is_64_bit_hypercall(vcpu);
 	if (longmode)
 		kvm_rax_write(vcpu, result);
 	else {
@@ -2171,7 +2171,7 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
 	}
 
 #ifdef CONFIG_X86_64
-	if (is_64_bit_mode(vcpu)) {
+	if (is_64_bit_hypercall(vcpu)) {
 		hc.param = kvm_rcx_read(vcpu);
 		hc.ingpa = kvm_rdx_read(vcpu);
 		hc.outgpa = kvm_r8_read(vcpu);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c85d70107057a4..ecd2041570d14a 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8848,7 +8848,7 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
 
 	trace_kvm_hypercall(nr, a0, a1, a2, a3);
 
-	op_64_bit = is_64_bit_mode(vcpu);
+	op_64_bit = is_64_bit_hypercall(vcpu);
 	if (!op_64_bit) {
 		nr &= 0xFFFFFFFF;
 		a0 &= 0xFFFFFFFF;
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index ea264c4502e413..997669ae9caa21 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -153,12 +153,24 @@ static inline bool is_64_bit_mode(struct kvm_vcpu *vcpu)
 {
 	int cs_db, cs_l;
 
+	WARN_ON_ONCE(vcpu->arch.guest_state_protected);
+
 	if (!is_long_mode(vcpu))
 		return false;
 	static_call(kvm_x86_get_cs_db_l_bits)(vcpu, &cs_db, &cs_l);
 	return cs_l;
 }
 
+static inline bool is_64_bit_hypercall(struct kvm_vcpu *vcpu)
+{
+	/*
+	 * If running with protected guest state, the CS register is not
+	 * accessible. The hypercall register values will have had to been
+	 * provided in 64-bit mode, so assume the guest is in 64-bit.
+	 */
+	return vcpu->arch.guest_state_protected || is_64_bit_mode(vcpu);
+}
+
 static inline bool x86_exception_has_error_code(unsigned int vector)
 {
 	static u32 exception_has_error_code = BIT(DF_VECTOR) | BIT(TS_VECTOR) |
diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index 565da9c3853bf4..dff2bdf9507a8c 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -698,7 +698,7 @@ int kvm_xen_hypercall(struct kvm_vcpu *vcpu)
 	    kvm_hv_hypercall_enabled(vcpu))
 		return kvm_hv_hypercall(vcpu);
 
-	longmode = is_64_bit_mode(vcpu);
+	longmode = is_64_bit_hypercall(vcpu);
 	if (!longmode) {
 		params[0] = (u32)kvm_rbx_read(vcpu);
 		params[1] = (u32)kvm_rcx_read(vcpu);