Hi Max,
On 5/24/14, 3:19 PM, Max Filippov wrote:
There are two FIXMEs in the double exception handler 'for the extremely
unlikely case'. This case gets hit by gcc during kernel build once in
a few hours, resulting in an unrecoverable exception condition.
So, it wasn't really that 'extremely' unlikely. Glad you found a case
and have a fix already! :-)
Just some minor questions/comments about the patch:
* Even with only a few instructions added to the double exception
handler, are we still in the limits of the vector (esp. if there are no
narrow instructions)? Does anyone know the current size limit?
* I guess you wanted to keep the vectors.S 'pure' to vector entries, and
added the fixup handler to the entry.S. I'm wondering if it would make
sense to keep the fixup closer to the cause, especially since ... (next
point)
* There seems to be some duplication of code between the overflow
handler and the fixup handler. Given that the fixup handler is really
rare, would it be possible to just jump into the original code?
* Minor (personal preferences) inline...
Thanks,
-Chris
Cc: [email protected]
Signed-off-by: Max Filippov <[email protected]>
---
arch/xtensa/kernel/entry.S | 109 ++++++++++++++++++++++++++++++++++++++++++
arch/xtensa/kernel/vectors.S | 46 ++++++++++--------
2 files changed, 134 insertions(+), 21 deletions(-)
diff --git a/arch/xtensa/kernel/entry.S b/arch/xtensa/kernel/entry.S
index ef7f499..c69b609 100644
--- a/arch/xtensa/kernel/entry.S
+++ b/arch/xtensa/kernel/entry.S
@@ -1400,6 +1400,115 @@ ENTRY(fast_syscall_spill_registers_fixup_return)
ENDPROC(fast_syscall_spill_registers_fixup_return)
+/*
+ * Fixup handler for TLB miss in double exception handler for window owerflow.
+ * We get here with windowbase set to the window that was being spilled and
+ * a0 trashed. a0 bit 7 determines if this is a call8 (bit clear) or call12
+ * (bit set) window.
+ *
+ * We do the following here:
+ * - go to the original window retaining a0 value;
+ * - set up exception stack to return back to appropriate a0 restore code
+ * (we'll need to rotate window back and there's no place to save this
+ * information, use different return address for that);
+ * - handle the exception;
+ * - go to the window that was being spilled;
+ * - set up window_overflow_restore_a0_fixup as a fixup routine;
+ * - reload a0;
+ * - restore the original window;
+ * - reset the default fixup routine;
+ * - return to user. By the time we get to this fixup handler all information
+ * about the conditions of the original double exception that happened in
+ * the window overflow handler is lost, so we just return to userspace to
+ * retry overflow from start.
+ *
+ * a0: value of depc, original value in depc
+ * a2: trashed, original value in EXC_TABLE_DOUBLE_SAVE
+ * a3: exctable, original value in excsave1
+ */
+
+ENTRY(window_overflow_restore_a0_fixup)
+
+ rsr a0, ps
+ extui a0, a0, PS_OWB_SHIFT, PS_OWB_WIDTH
+ rsr a2, windowbase
+ sub a0, a2, a0
+ extui a0, a0, 0, 3
+ l32i a2, a3, EXC_TABLE_DOUBLE_SAVE
+ xsr a3, excsave1
+
+ _beqi a0, 1, .L1pane
+ _beqi a0, 3, .L3pane
+
+ .macro overflow_fixup_handle_exception_pane n
+.L\n\()pane:
+ xsr a0, depc
Maybe use rsr instead of xsr as depc is not actually restored but
overwritten a few instructions later.
+ rotw -\n
+
+ xsr a3, excsave1
+ wsr a2, depc
+ l32i a2, a3, EXC_TABLE_KSTK
+ s32i a0, a2, PT_AREG0
+
+ movi a0, .L\n\()restore
+ s32i a0, a2, PT_DEPC
+
+ rsr a0, exccause
+ addx4 a0, a0, a3
+ l32i a0, a0, EXC_TABLE_FAST_USER
+ xsr a3, excsave1
+ jx a0
+
+ .endm
+
+ .macro overflow_fixup_restore_a0_pane n
+
+.L\n\()restore:
+ rotw \n
+ /* Need to preserve a0 value here to be able to handle exception
+ * that may occur on a0 reload from stack. It may occur because
+ * TLB miss handler may not be atomic and pointer to page table
+ * may be lost before we get here. There are no free registers,
+ * so we need to use EXC_TABLE_DOUBLE_SAVE area.
+ */
+ xsr a3, excsave1
+ s32i a2, a3, EXC_TABLE_DOUBLE_SAVE
+ movi a2, window_overflow_restore_a0_fixup
+ s32i a2, a3, EXC_TABLE_FIXUP
+ l32i a2, a3, EXC_TABLE_DOUBLE_SAVE
+ xsr a3, excsave1
+ bbsi.l a0, 7, 1f
+ l32e a0, a9, -16
+ j 2f
+1:
+ l32e a0, a13, -16
+2:
+ rotw -\n
+
+ .endm
+
+ overflow_fixup_handle_exception_pane 2
+ overflow_fixup_handle_exception_pane 1
+ overflow_fixup_handle_exception_pane 3
+
+ overflow_fixup_restore_a0_pane 2
+
+.Lset_default_fixup:
+ xsr a3, excsave1
+ s32i a2, a3, EXC_TABLE_DOUBLE_SAVE
+ movi a2, 0
+ s32i a2, a3, EXC_TABLE_FIXUP
+ l32i a2, a3, EXC_TABLE_DOUBLE_SAVE
+ xsr a3, excsave1
+ rfe
+
+ overflow_fixup_restore_a0_pane 1
+ j .Lset_default_fixup
+ overflow_fixup_restore_a0_pane 3
+ j .Lset_default_fixup
+
+ENDPROC(window_overflow_restore_a0_fixup)
+
#ifdef CONFIG_MMU
/*
* We should never get here. Bail out!
diff --git a/arch/xtensa/kernel/vectors.S b/arch/xtensa/kernel/vectors.S
index f9e1ec3..13c115f 100644
--- a/arch/xtensa/kernel/vectors.S
+++ b/arch/xtensa/kernel/vectors.S
@@ -376,38 +376,42 @@ _DoubleExceptionVector_WindowOverflow:
beqz a2, 1f # if at start of vector, don't restore
addi a0, a0, -128
- bbsi a0, 8, 1f # don't restore except for overflow 8 and 12
- bbsi a0, 7, 2f
+ bbsi.l a0, 8, 1f # don't restore except for overflow 8 and 12
+
+ /*
+ * This fixup handler is for the extremely unlikely case where the
+ * overflow handler's reference thru a0 gets a hardware TLB refill
+ * that bumps out the (distinct, aliasing) TLB entry that mapped its
+ * prior references thru a9/a13, and where our reference now thru
+ * a9/a13 gets a 2nd-level miss exception (not hardware TLB refill).
+ */
+ movi a2, window_overflow_restore_a0_fixup
+ s32i a2, a3, EXC_TABLE_FIXUP
+ l32i a2, a3, EXC_TABLE_DOUBLE_SAVE
+ xsr a3, excsave1
+
+ bbsi.l a0, 7, 2f
/*
* Restore a0 as saved by _WindowOverflow8().
- *
- * FIXME: we really need a fixup handler for this L32E,
- * for the extremely unlikely case where the overflow handler's
- * reference thru a0 gets a hardware TLB refill that bumps out
- * the (distinct, aliasing) TLB entry that mapped its prior
- * references thru a9, and where our reference now thru a9
- * gets a 2nd-level miss exception (not hardware TLB refill).
*/
- l32e a2, a9, -16
- wsr a2, depc # replace the saved a0
- j 1f
+ l32e a0, a9, -16
+ wsr a0, depc # replace the saved a0
+ j 3f
2:
/*
* Restore a0 as saved by _WindowOverflow12().
- *
- * FIXME: we really need a fixup handler for this L32E,
- * for the extremely unlikely case where the overflow handler's
- * reference thru a0 gets a hardware TLB refill that bumps out
- * the (distinct, aliasing) TLB entry that mapped its prior
- * references thru a13, and where our reference now thru a13
- * gets a 2nd-level miss exception (not hardware TLB refill).
*/
- l32e a2, a13, -16
- wsr a2, depc # replace the saved a0
+ l32e a0, a13, -16
+ wsr a0, depc # replace the saved a0
+3:
+ xsr a3, excsave1
+ movi a0, 0
+ s32i a0, a3, EXC_TABLE_FIXUP
+ s32i a2, a3, EXC_TABLE_DOUBLE_SAVE
1:
/*
* Restore WindowBase while leaving all address registers restored.
--
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