@@ -4334,6 +4334,16 @@ static void do_v7m_exception_exit(CPUARMState *env)
env->regs[12] = v7m_pop(env);
env->regs[14] = v7m_pop(env);
env->regs[15] = v7m_pop(env);
+ if (env->regs[15] & 1) {
+ qemu_log_mask(LOG_GUEST_ERROR,
+ "M profile return from interrupt with misaligned "
+ "PC is UNPREDICTABLE\n");
+ /* Actual hardware seems to ignore the lsbit, and there are several
+ * RTOSes out there which incorrectly assume the r15 in the stack
+ * frame should be a Thumb-style "lsbit indicates ARM/Thumb" value.
+ */
+ env->regs[15] &= ~1U;
+ }
xpsr = v7m_pop(env);
xpsr_write(env, xpsr, 0xfffffdff);
/* Undo stack alignment. */
For the ARM M-profile cores, exception return pops various registers including the PC from the stack. The architecture defines that if the lowest bit in the new PC value is set (ie the PC is not halfword aligned) then behaviour is UNPREDICTABLE. In practice hardware implementations seem to simply ignore the low bit, and some buggy RTOSes incorrectly rely on this. QEMU's behaviour was architecturally permitted, but bringing QEMU into line with the hardware behaviour allows more guest code to run. We log the situation as a guest error. This was reported as LP:1428657. Reported-by: Anders Esbensen <anders@lyes.dk> Signed-off-by: Peter Maydell <peter.maydell@linaro.org> --- target-arm/helper.c | 10 ++++++++++ 1 file changed, 10 insertions(+)