[kvm-unit-tests PATCH v3 4/7] s390x: Provide preliminary backtrace support

[Janosch Frank <frankja@xxxxxxxxxxxxx>]

After the stack changes we can finally use -mbackchain and have a
working backtrace.

Signed-off-by: Janosch Frank <frankja@xxxxxxxxxxxxx>
---
 lib/s390x/stack.c | 20 ++++++++++++++------
 s390x/Makefile    |  1 +
 s390x/macros.S    |  5 +++++
 3 files changed, 20 insertions(+), 6 deletions(-)

diff --git a/lib/s390x/stack.c b/lib/s390x/stack.c
index 0fcd1afb..4cf80dae 100644
--- a/lib/s390x/stack.c
+++ b/lib/s390x/stack.c
@@ -3,24 +3,32 @@
  * s390x stack implementation
  *
  * Copyright (c) 2017 Red Hat Inc
+ * Copyright 2021 IBM Corp
  *
  * Authors:
  *  Thomas Huth <thuth@xxxxxxxxxx>
  *  David Hildenbrand <david@xxxxxxxxxx>
+ *  Janosch Frank <frankja@xxxxxxxxxx>
  */
 #include <libcflat.h>
 #include <stack.h>
+#include <asm/arch_def.h>
 
 int backtrace_frame(const void *frame, const void **return_addrs, int max_depth)
 {
-	printf("TODO: Implement backtrace_frame(%p, %p, %d) function!\n",
-	       frame, return_addrs, max_depth);
-	return 0;
+	int depth = 0;
+	struct stack_frame *stack = (struct stack_frame *)frame;
+
+	for (depth = 0; stack && depth < max_depth; depth++) {
+		return_addrs[depth] = (void *)stack->grs[8];
+		stack = stack->back_chain;
+	}
+
+	return depth;
 }
 
 int backtrace(const void **return_addrs, int max_depth)
 {
-	printf("TODO: Implement backtrace(%p, %d) function!\n",
-	       return_addrs, max_depth);
-	return 0;
+	return backtrace_frame(__builtin_frame_address(0),
+			       return_addrs, max_depth);
 }
diff --git a/s390x/Makefile b/s390x/Makefile
index f3b0fccf..20bb5683 100644
--- a/s390x/Makefile
+++ b/s390x/Makefile
@@ -39,6 +39,7 @@ CFLAGS += -ffreestanding
 CFLAGS += -I $(SRCDIR)/lib -I $(SRCDIR)/lib/s390x -I lib
 CFLAGS += -O2
 CFLAGS += -march=zEC12
+CFLAGS += -mbackchain
 CFLAGS += -fno-delete-null-pointer-checks
 LDFLAGS += -nostdlib -Wl,--build-id=none
 
diff --git a/s390x/macros.S b/s390x/macros.S
index 11f4397a..d4f41ec4 100644
--- a/s390x/macros.S
+++ b/s390x/macros.S
@@ -22,6 +22,11 @@
 	lgr     %r2, %r15
 	/* Allocate stack space for called C function, as specified in s390 ELF ABI */
 	slgfi   %r15, 160
+	/*
+	 * Save the address of the interrupt stack into the back chain
+	 * of the called function.
+	 */
+	stg     %r2, STACK_FRAME_INT_BACKCHAIN(%r15)
 	brasl	%r14, \c_func
 	algfi   %r15, 160
 	RESTORE_REGS_STACK
-- 
2.25.1