[PATCH v2 02/13] xtensa: keep exception/interrupt stack continuous

[Max Filippov <jcmvbkbc@gmail.com>]

Restore original a0 in the kernel exception stack frame. This way it
looks like the frame that got interrupt/exception did alloca (copy a0 and
a1 potentially spilled under old stack to the new location as well) to
save registers and then did a call to handler. The point where
interrupt/exception was taken is not in the stack chain, only in pt_regs
(call4 from that address can be simulated to keep it in the stack trace).

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
---
Changes v1->v2:
- make continuous stack changes conditional;
- replace l32i/s32i + add used to access spilled registers with l32e/s32e.

 arch/xtensa/kernel/entry.S | 59 ++++++++++++++++++++++++++++++++++------------
 1 file changed, 44 insertions(+), 15 deletions(-)

diff --git a/arch/xtensa/kernel/entry.S b/arch/xtensa/kernel/entry.S
index 82bbfa5..b64075f 100644
--- a/arch/xtensa/kernel/entry.S
+++ b/arch/xtensa/kernel/entry.S
@@ -28,6 +28,10 @@
 #include <asm/tlbflush.h>
 #include <variant/tie-asm.h>
 
+#if defined(CONFIG_DEBUG_KERNEL) || defined(CONFIG_STACKTRACE_SUPPORT)
+#define XTENSA_CONTINUOUS_STACK
+#endif
+
 /* Unimplemented features. */
 
 #undef KERNEL_STACK_OVERFLOW_CHECK
@@ -219,6 +223,10 @@ _user_exception:
 
 2:	/* Now, jump to the common exception handler. */
 
+#ifdef XTENSA_CONTINUOUS_STACK
+	movi	a0, 0			# terminate user stack trace with 0
+	wsr	a0, depc
+#endif
 	j	common_exception
 
 ENDPROC(user_exception)
@@ -264,6 +272,24 @@ ENTRY(kernel_exception)
 	.globl _kernel_exception
 _kernel_exception:
 
+	/* Copy spill slots of a0 and a1 to imitate movsp
+	 * in order to keep exception stack continuous
+	 */
+#ifdef XTENSA_CONTINUOUS_STACK
+	/* Reload previous stack pointer: l32e relative to the current stack
+	 * won't reach it, because kernel exception stack frame is definitely
+	 * larger than l32e range (64 bytes).
+	 */
+	l32i	a0, a1, PT_AREG1
+	l32e	a3, a0, -16
+	l32e	a0, a0, -12
+	s32e	a3, a1, -16
+	s32e	a0, a1, -12
+
+	l32i	a0, a1, PT_AREG0	# restore saved a0
+	wsr	a0, depc
+#endif
+
 	/* Save SAR and turn off single stepping */
 
 	movi	a2, 0
@@ -346,12 +372,12 @@ common_exception:
 	s32i	a0, a1, PT_EXCCAUSE
 	s32i	a3, a2, EXC_TABLE_FIXUP
 
-	/* All unrecoverable states are saved on stack, now, and a1 is valid,
-	 * so we can allow exceptions and interrupts (*) again.
-	 * Set PS(EXCM = 0, UM = 0, RING = 0, OWB = 0, WOE = 1, INTLEVEL = X)
+	/* All unrecoverable states are saved on stack, now, and a1 is valid.
+	 * Now we can allow exceptions again. In case we've got an interrupt
+	 * PS.INTLEVEL is set to LOCKLEVEL disabling furhter interrupts,
+	 * otherwise it's left unchanged.
 	 *
-	 * (*) We only allow interrupts if they were previously enabled and
-	 *     we're not handling an IRQ
+	 * Set PS(EXCM = 0, UM = 0, RING = 0, OWB = 0, WOE = 1, INTLEVEL = X)
 	 */
 
 	rsr	a3, ps
@@ -362,28 +388,32 @@ common_exception:
 	moveqz	a3, a2, a0		# a3 = LOCKLEVEL iff interrupt
 	movi	a2, 1 << PS_WOE_BIT
 	or	a3, a3, a2
-	rsr	a0, exccause
+	rsr	a2, exccause
+#ifdef XTENSA_CONTINUOUS_STACK
+	/* restore return address (or 0 if return to userspace) */
+	rsr	a0, depc
+#endif
 	xsr	a3, ps
 
 	s32i	a3, a1, PT_PS		# save ps
 
 	/* Save lbeg, lend */
 
-	rsr	a2, lbeg
+	rsr	a4, lbeg
 	rsr	a3, lend
-	s32i	a2, a1, PT_LBEG
+	s32i	a4, a1, PT_LBEG
 	s32i	a3, a1, PT_LEND
 
 	/* Save SCOMPARE1 */
 
 #if XCHAL_HAVE_S32C1I
-	rsr     a2, scompare1
-	s32i    a2, a1, PT_SCOMPARE1
+	rsr     a3, scompare1
+	s32i    a3, a1, PT_SCOMPARE1
 #endif
 
 	/* Save optional registers. */
 
-	save_xtregs_opt a1 a2 a4 a5 a6 a7 PT_XTREGS_OPT
+	save_xtregs_opt a1 a3 a4 a5 a6 a7 PT_XTREGS_OPT
 	
 #ifdef CONFIG_TRACE_IRQFLAGS
 	l32i	a4, a1, PT_DEPC
@@ -391,8 +421,7 @@ common_exception:
 	 * while PS.EXCM was set, i.e. interrupts disabled.
 	 */
 	bgeui	a4, VALID_DOUBLE_EXCEPTION_ADDRESS, 1f
-	l32i	a4, a1, PT_EXCCAUSE
-	bnei	a4, EXCCAUSE_LEVEL1_INTERRUPT, 1f
+	bnei	a2, EXCCAUSE_LEVEL1_INTERRUPT, 1f
 	/* We came here with an interrupt means interrupts were enabled
 	 * and we've just disabled them.
 	 */
@@ -407,8 +436,8 @@ common_exception:
 
 	rsr	a4, excsave1
 	mov	a6, a1			# pass stack frame
-	mov	a7, a0			# pass EXCCAUSE
-	addx4	a4, a0, a4
+	mov	a7, a2			# pass EXCCAUSE
+	addx4	a4, a2, a4
 	l32i	a4, a4, EXC_TABLE_DEFAULT		# load handler
 
 	/* Call the second-level handler */
-- 
1.8.1.4