target-arm: Fix resetting issues on ARMv7-M CPUs

When calling qemu_system_reset after startup on a Cortex-M
CPU, the initial values of PC, MSP and the Thumb bit weren't being set
correctly if the vector table was in ROM. In particular, since Thumb was 0, a
Usage Fault would arise immediately after trying to execute any instruction
on a Cortex-M.
Signed-off-by: NMartin Galvan <martin.galvan@tallertechnologies.com>
Message-id: CAOKbPbaLt-LJsAKkQdOE0cs9Xx4OWrUfpDhATXPSdtuNw2xu_A@mail.gmail.com
[PMM: removed an incorrect comment]
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

target-arm/cpu.c

@@ -129,22 +129,34 @@ static void arm_cpu_reset(CPUState *s)
     env->uncached_cpsr = ARM_CPU_MODE_SVC;
     env->daif = PSTATE_D | PSTATE_A | PSTATE_I | PSTATE_F;
     /* On ARMv7-M the CPSR_I is the value of the PRIMASK register, and is
-       clear at reset.  Initial SP and PC are loaded from ROM.  */
+     * clear at reset. Initial SP and PC are loaded from ROM.
+     */
     if (IS_M(env)) {
-        uint32_t pc;
+        uint32_t initial_msp; /* Loaded from 0x0 */
+        uint32_t initial_pc; /* Loaded from 0x4 */
         uint8_t *rom;
+
         env->daif &= ~PSTATE_I;
         rom = rom_ptr(0);
         if (rom) {
-            /* We should really use ldl_phys here, in case the guest
-               modified flash and reset itself.  However images
-               loaded via -kernel have not been copied yet, so load the
-               values directly from there.  */
-            env->regs[13] = ldl_p(rom) & 0xFFFFFFFC;
-            pc = ldl_p(rom + 4);
-            env->thumb = pc & 1;
-            env->regs[15] = pc & ~1;
+            /* Address zero is covered by ROM which hasn't yet been
+             * copied into physical memory.
+             */
+            initial_msp = ldl_p(rom);
+            initial_pc = ldl_p(rom + 4);
+        } else {
+            /* Address zero not covered by a ROM blob, or the ROM blob
+             * is in non-modifiable memory and this is a second reset after
+             * it got copied into memory. In the latter case, rom_ptr
+             * will return a NULL pointer and we should use ldl_phys instead.
+             */
+            initial_msp = ldl_phys(s->as, 0);
+            initial_pc = ldl_phys(s->as, 4);
         }
+
+        env->regs[13] = initial_msp & 0xFFFFFFFC;
+        env->regs[15] = initial_pc & ~1;
+        env->thumb = initial_pc & 1;
     }
 
     if (env->cp15.c1_sys & SCTLR_V) {