If you try to build aarch64-linux-user with clang and --enable-debug then it
fails to compile:

 ld: libqemu-aarch64-linux-user.a.p/target_arm_cpu64.c.o: in function `cpu_arm_set_sve':
 ../../target/arm/cpu64.c:321:(.text+0x1254): undefined reference to `kvm_arm_sve_supported'

This is a regression introduced in commit f86d4220, which switched
the kvm-stub.c file away from being built for all arm targets to only
being built for system emulation binaries.  It doesn't affect gcc,
presumably because even at -O0 gcc folds away the always-false
kvm_enabled() condition but clang does not.

We would prefer not to build kvm-stub.c once for usermode and once
for system-emulation binaries, and we can't build it just once for
both because it includes cpu.h.  So instead provide always-false
versions of the five functions that are valid to call without KVM
support in kvm_arm.h.

Fixes: f86d42205c2eba ("target/arm/meson: accelerator files are not needed in user mode")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3033
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Message-id: 20250714135152.1896214-1-peter.maydell@linaro.org
---
 target/arm/kvm_arm.h | 35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/target/arm/kvm_arm.h b/target/arm/kvm_arm.h
index b4cad051551..6a9b6374a6d 100644
--- a/target/arm/kvm_arm.h
+++ b/target/arm/kvm_arm.h
@@ -161,6 +161,14 @@ void kvm_arm_add_vcpu_properties(ARMCPU *cpu);
  */
 void kvm_arm_steal_time_finalize(ARMCPU *cpu, Error **errp);
 
+/*
+ * These "is some KVM subfeature enabled?" functions may be called
+ * when KVM support is not present, including in the user-mode
+ * emulators. The kvm-stub.c file is only built into the system
+ * emulators, so for user-mode emulation we provide "always false"
+ * stubs here.
+ */
+#ifndef CONFIG_USER_ONLY
 /**
  * kvm_arm_aarch32_supported:
  *
@@ -197,6 +205,33 @@ bool kvm_arm_mte_supported(void);
  * Returns true if KVM can enable EL2 and false otherwise.
  */
 bool kvm_arm_el2_supported(void);
+#else
+
+static inline bool kvm_arm_aarch32_supported(void)
+{
+    return false;
+}
+
+static inline bool kvm_arm_pmu_supported(void)
+{
+    return false;
+}
+
+static inline bool kvm_arm_sve_supported(void)
+{
+    return false;
+}
+
+static inline bool kvm_arm_mte_supported(void)
+{
+    return false;
+}
+
+static inline bool kvm_arm_el2_supported(void)
+{
+    return false;
+}
+#endif
 
 /**
  * kvm_arm_get_max_vm_ipa_size:
-- 
2.43.0