[PATCH 3.13 155/259] xtensa: add fixup for double exception raised in window overflow

From: Kamal Mostafa
Date: Fri Aug 08 2014 - 16:44:17 EST

3.13.11.6 -stable review patch. If anyone has any objections, please let me know.

------------------

From: Max Filippov <jcmvbkbc@xxxxxxxxx>

commit 17290231df16eeee5dfc198dbf5ee4b419996dcd upstream.

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.

Provide missing fixup routine to handle this case. Double exception
literals now need 8 more bytes, add them to the linker script.

Also replace bbsi instructions with bbsi.l as we're branching depending
on 8th and 7th LSB-based bits of exception address.

This may be tested by adding the explicit DTLB invalidation to window
overflow handlers, like the following:

--- a/arch/xtensa/kernel/vectors.S
+++ b/arch/xtensa/kernel/vectors.S
@@ -592,6 +592,14 @@ ENDPROC(_WindowUnderflow4)
ENTRY_ALIGN64(_WindowOverflow8)

s32e a0, a9, -16
+ bbsi.l a9, 31, 1f
+ rsr a0, ccount
+ bbsi.l a0, 4, 1f
+ pdtlb a0, a9
+ idtlb a0
+ movi a0, 9
+ idtlb a0
+1:
l32e a0, a1, -12
s32e a2, a9, -8
s32e a1, a9, -12

Signed-off-by: Max Filippov <jcmvbkbc@xxxxxxxxx>
Signed-off-by: Kamal Mostafa <kamal@xxxxxxxxxxxxx>
---
arch/xtensa/kernel/vectors.S | 158 +++++++++++++++++++++++++++++++++------
arch/xtensa/kernel/vmlinux.lds.S | 4 +-
2 files changed, 138 insertions(+), 24 deletions(-)

diff --git a/arch/xtensa/kernel/vectors.S b/arch/xtensa/kernel/vectors.S
index cb8fd44..da0224d 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.
@@ -449,6 +453,7 @@ _DoubleExceptionVector_WindowOverflow:

s32i a0, a2, PT_DEPC

+_DoubleExceptionVector_handle_exception:
addx4 a0, a0, a3
l32i a0, a0, EXC_TABLE_FAST_USER
xsr a3, excsave1
@@ -464,11 +469,120 @@ _DoubleExceptionVector_WindowOverflow:
rotw -3
j 1b

- .end literal_prefix

ENDPROC(_DoubleExceptionVector)

/*
+ * 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, .Lhandle_1
+ _beqi a0, 3, .Lhandle_3
+
+ .macro overflow_fixup_handle_exception_pane n
+
+ rsr a0, depc
+ rotw -\n
+
+ xsr a3, excsave1
+ wsr a2, depc
+ l32i a2, a3, EXC_TABLE_KSTK
+ s32i a0, a2, PT_AREG0
+
+ movi a0, .Lrestore_\n
+ s32i a0, a2, PT_DEPC
+ rsr a0, exccause
+ j _DoubleExceptionVector_handle_exception
+
+ .endm
+
+ overflow_fixup_handle_exception_pane 2
+.Lhandle_1:
+ overflow_fixup_handle_exception_pane 1
+.Lhandle_3:
+ overflow_fixup_handle_exception_pane 3
+
+ .macro overflow_fixup_restore_a0_pane n
+
+ 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
+
+.Lrestore_2:
+ 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
+
+.Lrestore_1:
+ overflow_fixup_restore_a0_pane 1
+ j .Lset_default_fixup
+.Lrestore_3:
+ overflow_fixup_restore_a0_pane 3
+ j .Lset_default_fixup
+
+ENDPROC(window_overflow_restore_a0_fixup)
+
+ .end literal_prefix
+/*
* Debug interrupt vector
*
* There is not much space here, so simply jump to another handler.
diff --git a/arch/xtensa/kernel/vmlinux.lds.S b/arch/xtensa/kernel/vmlinux.lds.S
index 21acd11..af84f8f 100644
--- a/arch/xtensa/kernel/vmlinux.lds.S
+++ b/arch/xtensa/kernel/vmlinux.lds.S
@@ -262,13 +262,13 @@ SECTIONS
.UserExceptionVector.literal)
SECTION_VECTOR (_DoubleExceptionVector_literal,
.DoubleExceptionVector.literal,
- DOUBLEEXC_VECTOR_VADDR - 16,
+ DOUBLEEXC_VECTOR_VADDR - 40,
SIZEOF(.UserExceptionVector.text),
.UserExceptionVector.text)
SECTION_VECTOR (_DoubleExceptionVector_text,
.DoubleExceptionVector.text,
DOUBLEEXC_VECTOR_VADDR,
- 32,
+ 40,
.DoubleExceptionVector.literal)

. = (LOADADDR( .DoubleExceptionVector.text ) + SIZEOF( .DoubleExceptionVector.text ) + 3) & ~ 3;
--
1.9.1

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