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authorPeter Maydell <peter.maydell@linaro.org>2015-03-16 12:30:47 +0000
committerPeter Maydell <peter.maydell@linaro.org>2015-03-16 12:30:47 +0000
commitfcf83ab103dce6d2951f24f48e30820e7dbb3622 (patch)
tree45fd7842e9c64372faca50d236807dce7b11c362
parentda3e53ddcb0ca924da97ca5a35605fc554aa3e05 (diff)
downloadqemu-arm-fcf83ab103dce6d2951f24f48e30820e7dbb3622.tar.gz
target-arm: Ignore low bit of PC in M-profile exception return
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>
-rw-r--r--target-arm/helper.c10
1 files changed, 10 insertions, 0 deletions
diff --git a/target-arm/helper.c b/target-arm/helper.c
index 7fe3d14773..10886c5281 100644
--- a/target-arm/helper.c
+++ b/target-arm/helper.c
@@ -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. */