[RFC PATCH] powerpc/64/interrupt: Temporarily save PPR on stack to fix register corruption due to SLB miss

[Nicholas Piggin <npiggin@gmail.com>]

This is a minimal stable kernel fix for the problem solved by
4c2de74cc869 ("powerpc/64: Interrupts save PPR on stack rather than
thread_struct"). Instead of changing the interrupt stack frame (which
causes a lot of churn), it moves the PPR value from the PACA save area
to an unused slot in the stack frame temporarily, and defers saving it
to thread_struct to later on when it is safe to take SLB misses.

Upstream kernels between 4.17-4.20 have this bug, so I propose this
patch for 4.19 stable.

Fixes: f384796c4 ("powerpc/mm: Add support for handling > 512TB address in SLB miss")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 arch/powerpc/include/asm/exception-64s.h | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/include/asm/exception-64s.h b/arch/powerpc/include/asm/exception-64s.h
index 35fb5b11955a..f0424c6fdeca 100644
--- a/arch/powerpc/include/asm/exception-64s.h
+++ b/arch/powerpc/include/asm/exception-64s.h
@@ -243,10 +243,22 @@
  * PPR save/restore macros used in exceptions_64s.S  
  * Used for P7 or later processors
  */
-#define SAVE_PPR(area, ra, rb)						\
+#define SAVE_PPR(area, ra)						\
+BEGIN_FTR_SECTION_NESTED(940)						\
+	ld	ra,area+EX_PPR(r13);	/* Read PPR from paca */	\
+	std	ra,RESULT(r1);		/* Store PPR in RESULT for now */ \
+END_FTR_SECTION_NESTED(CPU_FTR_HAS_PPR,CPU_FTR_HAS_PPR,940)
+
+/*
+ * This is called after we are finished accessing 'area', so we can now take
+ * SLB faults accessing the thread struct, which will use PACA_EXSLB area.
+ * This is required because the large_addr_slb handler uses EXSLB and it also
+ * uses the common exception macros including this PPR saving.
+ */
+#define MOVE_PPR_TO_THREAD(ra, rb)					\
 BEGIN_FTR_SECTION_NESTED(940)						\
 	ld	ra,PACACURRENT(r13);					\
-	ld	rb,area+EX_PPR(r13);	/* Read PPR from paca */	\
+	ld	rb,RESULT(r1);		/* Read PPR from stack */	\
 	std	rb,TASKTHREADPPR(ra);					\
 END_FTR_SECTION_NESTED(CPU_FTR_HAS_PPR,CPU_FTR_HAS_PPR,940)
 
@@ -515,9 +527,11 @@ END_FTR_SECTION_NESTED(ftr,ftr,943)
 3:	EXCEPTION_PROLOG_COMMON_1();					   \
 	beq	4f;			/* if from kernel mode		*/ \
 	ACCOUNT_CPU_USER_ENTRY(r13, r9, r10);				   \
-	SAVE_PPR(area, r9, r10);					   \
+	SAVE_PPR(area, r9);						   \
 4:	EXCEPTION_PROLOG_COMMON_2(area)					   \
-	EXCEPTION_PROLOG_COMMON_3(n)					   \
+	beq	5f;			/* if from kernel mode		*/ \
+	MOVE_PPR_TO_THREAD(r9, r10);					   \
+5:	EXCEPTION_PROLOG_COMMON_3(n)					   \
 	ACCOUNT_STOLEN_TIME
 
 /* Save original regs values from save area to stack frame. */
-- 
2.23.0