samples: ftrace: Save required argument registers in sample trampolines

The ftrace-direct-too sample traces the handle_mm_fault function whose
signature changed since the introduction of the sample. Since:
commit bce617e ("mm: do page fault accounting in handle_mm_fault")
handle_mm_fault now has 4 arguments. Therefore, the sample trampoline
should save 4 argument registers.

s390 saves all argument registers already so it does not need a change
but x86_64 needs an extra push and pop.

This also evolves the signature of the tracing function to make it
mirror the signature of the traced function.

Link: https://lkml.kernel.org/r/[email protected]

Cc: [email protected]
Fixes: bce617e ("mm: do page fault accounting in handle_mm_fault")
Reviewed-by: Steven Rostedt (Google) <[email protected]>
Reviewed-by: Mark Rutland <[email protected]>
Acked-by: Catalin Marinas <[email protected]>
Signed-off-by: Florent Revest <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

samples/ftrace/ftrace-direct-too.c

@@ -5,14 +5,14 @@
 #include <linux/ftrace.h>
 #include <asm/asm-offsets.h>
 
-extern void my_direct_func(struct vm_area_struct *vma,
-			   unsigned long address, unsigned int flags);
+extern void my_direct_func(struct vm_area_struct *vma, unsigned long address,
+			   unsigned int flags, struct pt_regs *regs);
 
-void my_direct_func(struct vm_area_struct *vma,
-			unsigned long address, unsigned int flags)
+void my_direct_func(struct vm_area_struct *vma, unsigned long address,
+		    unsigned int flags, struct pt_regs *regs)
 {
-	trace_printk("handle mm fault vma=%p address=%lx flags=%x\n",
-		     vma, address, flags);
+	trace_printk("handle mm fault vma=%p address=%lx flags=%x regs=%p\n",
+		     vma, address, flags, regs);
 }
 
 extern void my_tramp(void *);
@@ -34,7 +34,9 @@ asm (
 "	pushq %rdi\n"
 "	pushq %rsi\n"
 "	pushq %rdx\n"
+"	pushq %rcx\n"
 "	call my_direct_func\n"
+"	popq %rcx\n"
 "	popq %rdx\n"
 "	popq %rsi\n"
 "	popq %rdi\n"