[kvm-unit-tests PATCH v5 18/29] powerpc: Fix stack backtrace termination

[Nicholas Piggin]

The backtrace handler terminates when it sees a NULL caller address,
but the powerpc stack setup does not keep such a NULL caller frame
at the start of the stack.

This happens to work on pseries because the memory at 0 is mapped and
it contains 0 at the location of the return address pointer if it
were a stack frame. But this is fragile, and does not work with powernv
where address 0 contains firmware instructions.

Use the existing dummy frame on stack as the NULL caller, and create a
new frame on stack for the entry code.

Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
---
 powerpc/cstart64.S | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/powerpc/cstart64.S b/powerpc/cstart64.S
index e18ae9a2..14ab0c6c 100644
--- a/powerpc/cstart64.S
+++ b/powerpc/cstart64.S
@@ -46,8 +46,16 @@ start:
 	add	r1, r1, r31
 	add	r2, r2, r31
 
+	/* Zero backpointers in initial stack frame so backtrace() stops */
+	li	r0,0
+	std	r0,0(r1)
+	std	r0,16(r1)
+
+	/* Create entry frame */
+	stdu	r1,-INT_FRAME_SIZE(r1)
+
 	/* save DTB pointer */
-	std	r3, 56(r1)
+	SAVE_GPR(3,r1)
 
 	/*
 	 * Call relocate. relocate is C code, but careful to not use
@@ -101,7 +109,7 @@ start:
 	stw	r4, 0(r3)
 
 	/* complete setup */
-1:	ld	r3, 56(r1)
+1:	REST_GPR(3, r1)
 	bl	setup
 
 	/* run the test */
-- 
2.42.0