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


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