xen: drop the nop() macro

There isn't a plausible reason to insert nops into code in this manner.

The sole use is in do_debug_key(), and exists to prevent the compiler
optimising the tail of the function with 'jmp debugger_trap_fatal'

In practice, a compiler barrier suffices just as well to prevent the tailcall,
and doesn't involve inserting unnecessary instructions.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <julien.grall@arm.com>

xen/common/keyhandler.c

@@ -480,9 +480,9 @@ static void do_debug_key(unsigned char key, struct cpu_user_regs *regs)
 {
     printk("'%c' pressed -> trapping into debugger\n", key);
     (void)debugger_trap_fatal(0xf001, regs);
-    nop(); /* Prevent the compiler doing tail call
-                             optimisation, as that confuses xendbg a
-                             bit. */
+
+    /* Prevent tail call optimisation, which confuses xendbg. */
+    barrier();
 }
 
 static void do_toggle_alt_key(unsigned char key, struct cpu_user_regs *regs)

xen/include/asm-arm/system.h

@@ -5,9 +5,6 @@
 #include <xen/lib.h>
 #include <public/arch-arm.h>
 
-#define nop() \
-    asm volatile ( "nop" )
-
 #define sev()           asm volatile("sev" : : : "memory")
 #define wfe()           asm volatile("wfe" : : : "memory")
 #define wfi()           asm volatile("wfi" : : : "memory")

xen/include/asm-x86/system.h

@@ -17,9 +17,6 @@
 #define clflush(a) \
     asm volatile ( "clflush (%0)" : : "r"(a) )
 
-#define nop() \
-    asm volatile ( "nop" )
-
 #define xchg(ptr,v) \
     ((__typeof__(*(ptr)))__xchg((unsigned long)(v),(ptr),sizeof(*(ptr))))