s390/kernel: emit CFI data in .debug_frame and discard .eh_frame sections

Using perf probe and libdw on kernel modules failed to find CFI
data for symbols.  The CFI data is stored in the .eh_frame section.
The elfutils libdw is not able to extract the CFI data correctly,
because the .eh_frame section requires "non-simple" relocations
for kernel modules.

The suggestion is to avoid these "non-simple" relocations by emitting
the CFI data in the .debug_frame section.  Let gcc emit respective
directives by specifying the -fno-asynchronous-unwind-tables option.

Using the .debug_frame section for CFI data, the .eh_frame section
becomes unused and, thus, discard it for kernel and modules builds

The vDSO requires the .eh_frame section and, hence, emit the CFI data
in both, the .eh_frame and .debug_frame sections.

See also discussion on elfutils/libdw bugzilla:
https://sourceware.org/bugzilla/show_bug.cgi?id=22452

Suggested-by: Mark Wielaard <mark@klomp.org>
Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/include/asm/dwarf.h

@@ -0,0 +1,25 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_S390_DWARF_H
+#define _ASM_S390_DWARF_H
+
+#ifdef __ASSEMBLY__
+
+#ifndef BUILD_VDSO
+	/*
+	 * Emit CFI data in .debug_frame sections and not in .eh_frame
+	 * sections.  The .eh_frame CFI is used for runtime unwind
+	 * information that is not being used.  Hence, vmlinux.lds.S
+	 * can discard the .eh_frame sections.
+	 */
+	.cfi_sections .debug_frame
+#else
+	/*
+	 * For vDSO, emit CFI data in both, .eh_frame and .debug_frame
+	 * sections.
+	 */
+	.cfi_sections .eh_frame, .debug_frame
+#endif
+
+#endif	/* __ASSEMBLY__ */
+
+#endif	/* _ASM_S390_DWARF_H */