comment possibly-confusing i386 vsyscall asm

src/internal/i386/syscall.s

 .hidden __sysinfo
 
+# The calling convention for __vsyscall has the syscall number
+# and 5 args arriving as:  eax, edx, ecx, edi, esi, 4(%esp).
+# This ensures that the inline asm in the C code never has to touch
+# ebx or ebp (which are unavailable in PIC and frame-pointer-using
+# code, respectively), and optimizes for size/simplicity in the caller.
+
 .global __vsyscall
 .type __vsyscall,@function
 __vsyscall:
@@ -22,11 +28,17 @@ __vsyscall:
 	jz 1f
 	push %eax
 	mov 8(%esp),%eax
-	ret
+	ret                     # tail call to kernel vsyscall entry
 1:	mov 4(%esp),%eax
 	int $128
 	ret
 
+# The __vsyscall6 entry point is used only for 6-argument syscalls.
+# Instead of passing the 5th argument on the stack, a pointer to the
+# 5th and 6th arguments is passed. This is ugly, but there are no
+# register constraints the inline asm could use that would make it
+# possible to pass two arguments on the stack.
+
 .global __vsyscall6
 .type __vsyscall6,@function
 __vsyscall6: