x86/altcall: Rename alternative_branches() to boot_apply_alt_calls()

The alternatives APIs are not great; rename alternative_branches() to be more
precise.  Centralise the declaration in xen/alternative-call.h, in the
expectation that x86 won't be the only user in the long term.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/alternative.c

@@ -493,7 +493,7 @@ void __init alternative_instructions(void)
     _alternative_instructions(false);
 }
 
-void __init alternative_branches(void)
+void __init boot_apply_alt_calls(void)
 {
     local_irq_disable();
     _alternative_instructions(true);

xen/arch/x86/include/asm/alternative.h

@@ -29,7 +29,6 @@ extern void add_nops(void *insns, unsigned int len);
 /* Similar to alternative_instructions except it can be run with IRQs enabled. */
 extern int apply_alternatives(struct alt_instr *start, struct alt_instr *end);
 extern void alternative_instructions(void);
-extern void alternative_branches(void);
 
 #define alt_orig_len       "(.LXEN%=_orig_e - .LXEN%=_orig_s)"
 #define alt_pad_len        "(.LXEN%=_orig_p - .LXEN%=_orig_e)"

xen/arch/x86/setup.c

@@ -1,4 +1,5 @@
 #include <xen/acpi.h>
+#include <xen/alternative-call.h>
 #include <xen/bitops.h>
 #include <xen/console.h>
 #include <xen/cpu.h>
@@ -2082,7 +2083,7 @@ void asmlinkage __init noreturn __start_xen(void)
 
     do_presmp_initcalls();
 
-    alternative_branches();
+    boot_apply_alt_calls();
 
     /*
      * NB: when running as a PV shim VCPUOP_up/down is wired to the shim

xen/include/xen/alternative-call.h

@@ -17,6 +17,8 @@
  *   generation requirements are to emit a function pointer call at build
  *   time, and stash enough metadata to simplify the call at boot once the
  *   implementation has been resolved.
+ * - Implement boot_apply_alt_calls() to convert the function pointer calls
+ *   into direct calls on boot.
  * - Select ALTERNATIVE_CALL in Kconfig.
  *
  * To use:
@@ -57,7 +59,13 @@
 # define __alt_call_maybe_initdata __initdata
 #endif
 
-#else
+/*
+ * Devirtualise the alternative_{,v}call()'s on boot.  Convert still-NULL
+ * function pointers into traps.
+ */
+void boot_apply_alt_calls(void);
+
+#else /* CONFIG_ALTERNATIVE_CALL */
 
 #define alternative_call(func, args...)  (func)(args)
 #define alternative_vcall(func, args...) (func)(args)