target-arm: Implement a minimal set of cp14 debug registers

Newer ARM kernels try to probe for whether the CPU has hardware breakpoint
support. For this to work QEMU has to implement a minimal set of the cp14
debug registers. The architecture requires v7 cores to implement debug
and so there is no defined way to report its absence; however in practice
returning a zero DBGDIDR (ie with a reserved value for "debug architecture
version") should cause well-written hw debug users to do the right thing.
We also implement DBGDRAR and DBGDSAR as RAZ, indicating no memory mapped
debug components.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

target-arm/translate.c

@@ -5778,6 +5778,34 @@ static int disas_cp14_read(CPUState * env, DisasContext *s, uint32_t insn)
     int rt = (insn >> 12) & 0xf;
     TCGv tmp;
 
+    /* Minimal set of debug registers, since we don't support debug */
+    if (op1 == 0 && crn == 0 && op2 == 0) {
+        switch (crm) {
+        case 0:
+            /* DBGDIDR: just RAZ. In particular this means the
+             * "debug architecture version" bits will read as
+             * a reserved value, which should cause Linux to
+             * not try to use the debug hardware.
+             */
+            tmp = tcg_const_i32(0);
+            store_reg(s, rt, tmp);
+            return 0;
+        case 1:
+        case 2:
+            /* DBGDRAR and DBGDSAR: v7 only. Always RAZ since we
+             * don't implement memory mapped debug components
+             */
+            if (ENABLE_ARCH_7) {
+                tmp = tcg_const_i32(0);
+                store_reg(s, rt, tmp);
+                return 0;
+            }
+            break;
+        default:
+            break;
+        }
+    }
+
     if (arm_feature(env, ARM_FEATURE_THUMB2EE)) {
         if (op1 == 6 && crn == 0 && crm == 0 && op2 == 0) {
             /* TEECR */