hvf: arm: Handle unknown ID registers as RES0

Recent Linux versions added support to read ID_AA64ISAR2_EL1. On M1,
those reads trap into QEMU which handles them as faults.

However, AArch64 ID registers should always read as RES0. Let's
handle them accordingly.

This fixes booting Linux 5.17 guests.

Cc: qemu-stable@nongnu.org
Reported-by: Ivan Babrou <ivan@cloudflare.com>
Signed-off-by: Alexander Graf <agraf@csgraf.de>
Message-id: 20220209124135.69183-2-agraf@csgraf.de
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

target/arm/hvf/hvf.c

@@ -754,6 +754,15 @@ static bool hvf_handle_psci_call(CPUState *cpu)
     return true;
 }
 
+static bool is_id_sysreg(uint32_t reg)
+{
+    return SYSREG_OP0(reg) == 3 &&
+           SYSREG_OP1(reg) == 0 &&
+           SYSREG_CRN(reg) == 0 &&
+           SYSREG_CRM(reg) >= 1 &&
+           SYSREG_CRM(reg) < 8;
+}
+
 static int hvf_sysreg_read(CPUState *cpu, uint32_t reg, uint32_t rt)
 {
     ARMCPU *arm_cpu = ARM_CPU(cpu);
@@ -806,6 +815,11 @@ static int hvf_sysreg_read(CPUState *cpu, uint32_t reg, uint32_t rt)
         /* Dummy register */
         break;
     default:
+        if (is_id_sysreg(reg)) {
+            /* ID system registers read as RES0 */
+            val = 0;
+            break;
+        }
         cpu_synchronize_state(cpu);
         trace_hvf_unhandled_sysreg_read(env->pc, reg,
                                         SYSREG_OP0(reg),