s390: implement perf_arch_fetch_caller_regs

On s390 bpf_get_stack_raw_tp() returns 0 entries for both kernel and
user stacks. While there is no practical unwinding solution for userspace
on s390 at this moment, there certainly is a kernel unwinder. However,
it is not properly integrated with BPF.

In order to start unwinding, bpf_get_stack_raw_tp() obtains the current
kernel register values using perf_fetch_caller_regs(), which is not
implemented for s390. The actual unwinding then happens by passing those
registers to perf_callchain_kernel().

Implement perf_arch_fetch_caller_regs() for s390, where
__builtin_frame_address(0) points to back_chain.
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/include/asm/perf_event.h

@@ -12,6 +12,7 @@
 
 #include <linux/perf_event.h>
 #include <linux/device.h>
+#include <asm/stacktrace.h>
 
 /* Per-CPU flags for PMU states */
 #define PMU_F_RESERVED			0x1000
@@ -73,4 +74,10 @@ struct perf_sf_sde_regs {
 #define SDB_FULL_BLOCKS(hwc)	(SAMPL_FLAGS(hwc) & PERF_CPUM_SF_FULL_BLOCKS)
 #define SAMPLE_FREQ_MODE(hwc)	(SAMPL_FLAGS(hwc) & PERF_CPUM_SF_FREQ_MODE)
 
+#define perf_arch_fetch_caller_regs(regs, __ip) do {			\
+	(regs)->psw.addr = (__ip);					\
+	(regs)->gprs[15] = (unsigned long)__builtin_frame_address(0) -	\
+		offsetof(struct stack_frame, back_chain);		\
+} while (0)
+
 #endif /* _ASM_S390_PERF_EVENT_H */