[Qemu-devel] [PATCH 2/6] armv7m: Replace armv7m.hack with unassigned_access handler

[Peter Maydell]

From: Michael Davidsaver <address@hidden>

For v7m we need to catch attempts to execute from special
addresses at 0xfffffff0 and above. Previously we did this
with the aid of a hacky special purpose lump of memory
in the address space and a check in translate.c for whether
we were translating code at those addresses.

We can implement this more cleanly using a CPU
unassigned access handler which throws the exception
if the unassigned access is for one of the special addresses.

Signed-off-by: Michael Davidsaver <address@hidden>
[PMM:
 * drop the deletion of the "don't interrupt if PC is magic"
   code in arm_v7m_cpu_exec_interrupt() -- this is still
   required
 * don't generate an exception for unassigned accesses
   which aren't to the magic address -- although doing
   this is in theory correct in practice it will break
   currently working guests which rely on the RAZ/WI
   behaviour when they touch devices which we haven't
   modelled.
 * trigger EXCP_EXCEPTION_EXIT on is_exec, not !is_write
]
Signed-off-by: Peter Maydell <address@hidden>
---
 hw/arm/armv7m.c        |  8 --------
 target/arm/cpu.c       | 28 ++++++++++++++++++++++++++++
 target/arm/translate.c | 12 ++++++------
 3 files changed, 34 insertions(+), 14 deletions(-)

diff --git a/hw/arm/armv7m.c b/hw/arm/armv7m.c
index 49d3078..0c9ca7b 100644
--- a/hw/arm/armv7m.c
+++ b/hw/arm/armv7m.c
@@ -180,7 +180,6 @@ DeviceState *armv7m_init(MemoryRegion *system_memory, int 
mem_size, int num_irq,
     uint64_t entry;
     uint64_t lowaddr;
     int big_endian;
-    MemoryRegion *hack = g_new(MemoryRegion, 1);
 
     if (cpu_model == NULL) {
        cpu_model = "cortex-m3";
@@ -225,13 +224,6 @@ DeviceState *armv7m_init(MemoryRegion *system_memory, int 
mem_size, int num_irq,
         }
     }
 
-    /* Hack to map an additional page of ram at the top of the address
-       space.  This stops qemu complaining about executing code outside RAM
-       when returning from an exception.  */
-    memory_region_init_ram(hack, NULL, "armv7m.hack", 0x1000, &error_fatal);
-    vmstate_register_ram_global(hack);
-    memory_region_add_subregion(system_memory, 0xfffff000, hack);
-
     qemu_register_reset(armv7m_reset, cpu);
     return nvic;
 }
diff --git a/target/arm/cpu.c b/target/arm/cpu.c
index 3f2cdb6..47759c9 100644
--- a/target/arm/cpu.c
+++ b/target/arm/cpu.c
@@ -292,6 +292,33 @@ bool arm_cpu_exec_interrupt(CPUState *cs, int 
interrupt_request)
 }
 
 #if !defined(CONFIG_USER_ONLY) || !defined(TARGET_AARCH64)
+static void arm_v7m_unassigned_access(CPUState *cpu, hwaddr addr,
+                                      bool is_write, bool is_exec, int opaque,
+                                      unsigned size)
+{
+    ARMCPU *arm = ARM_CPU(cpu);
+    CPUARMState *env = &arm->env;
+
+    /* ARMv7-M interrupt return works by loading a magic value into the PC.
+     * On real hardware the load causes the return to occur.  The qemu
+     * implementation performs the jump normally, then does the exception
+     * return by throwing a special exception when when the CPU tries to
+     * execute code at the magic address.
+     */
+    if (env->v7m.exception != 0 && addr >= 0xfffffff0 && is_exec) {
+        cpu->exception_index = EXCP_EXCEPTION_EXIT;
+        cpu_loop_exit(cpu);
+    }
+
+    /* In real hardware an attempt to access parts of the address space
+     * with nothing there will usually cause an external abort.
+     * However our QEMU board models are often missing device models where
+     * the guest can boot anyway with the default read-as-zero/writes-ignored
+     * behaviour that you get without a QEMU unassigned_access hook.
+     * So just return here to retain that default behaviour.
+     */
+}
+
 static bool arm_v7m_cpu_exec_interrupt(CPUState *cs, int interrupt_request)
 {
     CPUClass *cc = CPU_GET_CLASS(cs);
@@ -1016,6 +1043,7 @@ static void arm_v7m_class_init(ObjectClass *oc, void 
*data)
     cc->do_interrupt = arm_v7m_cpu_do_interrupt;
 #endif
 
+    cc->do_unassigned_access = arm_v7m_unassigned_access;
     cc->cpu_exec_interrupt = arm_v7m_cpu_exec_interrupt;
 }
 
diff --git a/target/arm/translate.c b/target/arm/translate.c
index c9186b6..a7c2abe 100644
--- a/target/arm/translate.c
+++ b/target/arm/translate.c
@@ -11719,12 +11719,12 @@ void gen_intermediate_code(CPUARMState *env, 
TranslationBlock *tb)
             break;
         }
 #else
-        if (dc->pc >= 0xfffffff0 && arm_dc_feature(dc, ARM_FEATURE_M)) {
-            /* We always get here via a jump, so know we are not in a
-               conditional execution block.  */
-            gen_exception_internal(EXCP_EXCEPTION_EXIT);
-            dc->is_jmp = DISAS_EXC;
-            break;
+        if (arm_dc_feature(dc, ARM_FEATURE_M)) {
+            /* Branches to the magic exception-return addresses should
+             * already have been caught via the arm_v7m_unassigned_access hook,
+             * and never get here.
+             */
+            assert(dc->pc < 0xfffffff0);
         }
 #endif
 
-- 
2.7.4