[PATCH] Fix exception handler for supporting FPU

[Martin Galvan]

On Thu, Aug 27, 2015 at 3:53 PM, sudarshan.rajagopalan
<sudarshan.rajagopalan at vecna.com> wrote:
> The instruction "cmn r2, #3\n" in line 31 basically compares the Link
> Register(lr) to value 0xFFFFFFFD (negative #3, because CMN negates the RHS
> and compares with LHS) and chooses MSP or PSP in the following IT block.
> This is pretty cool! But, it will not work if you have the floating-point
> unit (FPU) enabled in your processor, which is enabled in mine. With FPU
> enabled, the error code returned is either 0xFFFFFFE9 or 0xFFFFFFED, for
> which the above assembly instruction will not work out and MSP will be
> selected always.
>
> Better way to do is to check the 2nd bit of the error code to determine
> which stack pointer was used before the exception happened - "tst lr, #4\n"
> and change the IT block from "itt ne" to "itt eq" and the "mov" and "add"
> within this IT block.

Hi Sudarshan! First of all, great catch. It looks like there's indeed a bug
on certain conditions.

While your method is indeed way less convoluted than the clever "compare to -3"
the old code did, I think we can actually make it even simpler
(and save a couple of instructions) by doing the following:

    "tst lr, #4\n" /* Check if bit 2 of LR is zero */
    "itte eq\n" /* IF bit 2 of LR is zero... */
    "mrseq r0, msp\n" /* THEN we were using MSP */
    "addeq r0, %[cpufsz]\n" /* THEN revert the previous 'sub sp, %[cpufsz]' */
    "mrsne r0, psp\n" /* ELSE it's not zero; we were using PSP */

What do you think?

Also, I must admit it took me quite a while to understand what was going on there.
If you don't mind, I'd also like to add a brief comment explaining how to decode LR
after taking an exception. The patch would end up looking like this:

---
 cpukit/score/cpu/arm/armv7m-exception-default.c | 27 +++++++++++++++++--------
 1 file changed, 19 insertions(+), 8 deletions(-)

diff --git a/cpukit/score/cpu/arm/armv7m-exception-default.c b/cpukit/score/cpu/arm/armv7m-exception-default.c
index e890cdf..d04dbea 100644
--- a/cpukit/score/cpu/arm/armv7m-exception-default.c
+++ b/cpukit/score/cpu/arm/armv7m-exception-default.c
@@ -22,16 +22,27 @@
 
 void __attribute__((naked)) _ARMV7M_Exception_default( void )
 {
+    /* On exception entry, ARMv7M saves context state onto a stack pointed to
+     * by either MSP or PSP. The value stored in LR indicates whether we were
+     * in Thread or Handler mode, whether we were using the FPU (if any),
+     * and which stack pointer we were using.
+     * In particular, bit 2 of LR will be 0 if we were using MSP.
+     *
+     * For a more detailed explanation, see the Exception Entry Behavior
+     * section of the ARMv7M Architecture Reference Manual.
+     */
+
+    /* As we're in Handler mode here, we'll always operate on MSP.
+     * However, we need to store the right SP in our CPU_Exception_frame.
+     */
   __asm__ volatile (
-    "sub sp, %[cpufsz]\n"
+    "sub sp, %[cpufsz]\n" /* Allocate space for a CPU_Exception_frame. */
     "stm sp, {r0-r12}\n"
-    "mov r2, lr\n"
-    "mrs r1, msp\n"
-    "mrs r0, psp\n"
-    "cmn r2, #3\n"
-    "itt ne\n"
-    "movne r0, r1\n"
-    "addne r0, %[cpufsz]\n"
+    "tst lr, #4\n" /* Check if bit 2 of LR is zero. (If so, PSR.Z = 1) */
+    "itte eq\n" /* IF bit 2 of LR is zero... (PSR.Z == 1) */
+    "mrseq r0, msp\n" /* THEN we were using MSP. */
+    "addeq r0, %[cpufsz]\n" /* THEN, set r0 = old MSP. */
+    "mrsne r0, psp\n" /* ELSE it's not zero; we were using PSP. */
     "add r2, r0, %[v7mlroff]\n"
     "add r1, sp, %[cpulroff]\n"
     "ldm r2, {r3-r5}\n"
-- 
1.9.1