Misaligned thumb PC is architecturally impossible.
Assert is better than proceeding, in case we've missed
something somewhere.

Expand a comment about aligning the pc in gdbstub.
Fail an incoming migrate if a thumb pc is misaligned.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
---
 target/arm/gdbstub.c   | 9 +++++++--
 target/arm/machine.c   | 9 +++++++++
 target/arm/translate.c | 3 +++
 3 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/target/arm/gdbstub.c b/target/arm/gdbstub.c
index 826601b341..a54b42418b 100644
--- a/target/arm/gdbstub.c
+++ b/target/arm/gdbstub.c
@@ -76,8 +76,13 @@ int arm_cpu_gdb_write_register(CPUState *cs, uint8_t *mem_buf, int n)
 
     tmp = ldl_p(mem_buf);
 
-    /* Mask out low bit of PC to workaround gdb bugs.  This will probably
-       cause problems if we ever implement the Jazelle DBX extensions.  */
+    /*
+     * Mask out low bits of PC to workaround gdb bugs.
+     * This avoids an assert in thumb_tr_translate_insn, because it is
+     * architecturally impossible to misalign the pc.
+     * This will probably cause problems if we ever implement the
+     * Jazelle DBX extensions.
+     */
     if (n == 15) {
         tmp &= ~1;
     }
diff --git a/target/arm/machine.c b/target/arm/machine.c
index 81e30de824..b5004a67e9 100644
--- a/target/arm/machine.c
+++ b/target/arm/machine.c
@@ -781,6 +781,15 @@ static int cpu_post_load(void *opaque, int version_id)
     hw_breakpoint_update_all(cpu);
     hw_watchpoint_update_all(cpu);
 
+    /*
+     * Misaligned thumb pc is architecturally impossible.
+     * We have an assert in thumb_tr_translate_insn to verify this.
+     * Fail an incoming migrate to avoid this assert.
+     */
+    if (!is_a64(env) && env->thumb && (env->regs[15] & 1)) {
+        return -1;
+    }
+
     if (!kvm_enabled()) {
         pmu_op_finish(&cpu->env);
     }
diff --git a/target/arm/translate.c b/target/arm/translate.c
index dfeaa2321d..a93ea3c47c 100644
--- a/target/arm/translate.c
+++ b/target/arm/translate.c
@@ -9595,6 +9595,9 @@ static void thumb_tr_translate_insn(DisasContextBase *dcbase, CPUState *cpu)
     uint32_t insn;
     bool is_16bit;
 
+    /* Misaligned thumb PC is architecturally impossible. */
+    assert((dc->base.pc_next & 1) == 0);
+
     if (arm_check_ss_active(dc) || arm_check_kernelpage(dc)) {
         dc->base.pc_next += 2;
         return;
-- 
2.25.1

On 8/21/21 9:59 PM, Richard Henderson wrote:
> Misaligned thumb PC is architecturally impossible.
> Assert is better than proceeding, in case we've missed
> something somewhere.
> 
> Expand a comment about aligning the pc in gdbstub.
> Fail an incoming migrate if a thumb pc is misaligned.
> 
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>  target/arm/gdbstub.c   | 9 +++++++--
>  target/arm/machine.c   | 9 +++++++++
>  target/arm/translate.c | 3 +++
>  3 files changed, 19 insertions(+), 2 deletions(-)

> diff --git a/target/arm/translate.c b/target/arm/translate.c
> index dfeaa2321d..a93ea3c47c 100644
> --- a/target/arm/translate.c
> +++ b/target/arm/translate.c
> @@ -9595,6 +9595,9 @@ static void thumb_tr_translate_insn(DisasContextBase *dcbase, CPUState *cpu)
>      uint32_t insn;
>      bool is_16bit;
>  
> +    /* Misaligned thumb PC is architecturally impossible. */
> +    assert((dc->base.pc_next & 1) == 0);

What about using tcg_debug_assert() instead?

>      if (arm_check_ss_active(dc) || arm_check_kernelpage(dc)) {
>          dc->base.pc_next += 2;
>          return;
> 

On 8/21/21 1:46 PM, Philippe Mathieu-Daudé wrote:
>> +    /* Misaligned thumb PC is architecturally impossible. */
>> +    assert((dc->base.pc_next & 1) == 0);
> 
> What about using tcg_debug_assert() instead?

I don't think we want to let this one compile out.


r~

On Sat, 21 Aug 2021 at 21:04, Richard Henderson
<richard.henderson@linaro.org> wrote:
>
> Misaligned thumb PC is architecturally impossible.
> Assert is better than proceeding, in case we've missed
> something somewhere.
>
> Expand a comment about aligning the pc in gdbstub.
> Fail an incoming migrate if a thumb pc is misaligned.
>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>

thanks
-- PMM