x86/APIC: finish genapic conversion to altcall

While .probe() doesn't need fiddling with for being run only very early,
init_apic_ldr() wants converting too despite not being on a frequently
executed path: This way all pre-filled struct genapic instances can
become __initconst_cf_clobber, thus allowing to eliminate 15 more ENDBR
during the 2nd phase of alternatives patching.

While fiddling with section annotations here, also move "genapic" itself
to .data.ro_after_init.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
(cherry picked from commit b1cc53753c)

xen/arch/x86/genapic/bigsmp.c

@@ -41,7 +41,7 @@ static int __init cf_check probe_bigsmp(void)
 	return def_to_bigsmp;
 } 
 
-const struct genapic __initconstrel apic_bigsmp = {
+const struct genapic __initconst_cf_clobber apic_bigsmp = {
 	APIC_INIT("bigsmp", probe_bigsmp),
 	GENAPIC_PHYS
 };

xen/arch/x86/genapic/default.c

@@ -14,7 +14,7 @@
 #include <asm/io_apic.h>
 
 /* should be called last. */
-const struct genapic __initconstrel apic_default = {
+const struct genapic __initconst_cf_clobber apic_default = {
 	APIC_INIT("default", NULL),
 	GENAPIC_FLAT
 };

xen/arch/x86/genapic/probe.c

@@ -16,7 +16,7 @@
 #include <asm/mach-generic/mach_apic.h>
 #include <asm/setup.h>
 
-struct genapic __read_mostly genapic;
+struct genapic __ro_after_init genapic;
 
 static const struct genapic *const __initconstrel apic_probe[] = {
 	&apic_bigsmp, 

xen/arch/x86/genapic/x2apic.c

@@ -169,7 +169,7 @@ static void cf_check send_IPI_mask_x2apic_cluster(
     local_irq_restore(flags);
 }
 
-static const struct genapic __initconstrel apic_x2apic_phys = {
+static const struct genapic __initconst_cf_clobber apic_x2apic_phys = {
     APIC_INIT("x2apic_phys", NULL),
     .int_delivery_mode = dest_Fixed,
     .int_dest_mode = 0 /* physical delivery */,
@@ -180,7 +180,7 @@ static const struct genapic __initconstrel apic_x2apic_phys = {
     .send_IPI_self = send_IPI_self_x2apic
 };
 
-static const struct genapic __initconstrel apic_x2apic_cluster = {
+static const struct genapic __initconst_cf_clobber apic_x2apic_cluster = {
     APIC_INIT("x2apic_cluster", NULL),
     .int_delivery_mode = dest_LowestPrio,
     .int_dest_mode = 1 /* logical delivery */,
@@ -198,7 +198,7 @@ static const struct genapic __initconstrel apic_x2apic_cluster = {
  * IPIs to be more efficiently delivered by not having to perform an ICR write
  * for each target CPU.
  */
-static const struct genapic __initconstrel apic_x2apic_mixed = {
+static const struct genapic __initconst_cf_clobber apic_x2apic_mixed = {
     APIC_INIT("x2apic_mixed", NULL),
 
     /*

xen/arch/x86/include/asm/mach-generic/mach_apic.h

@@ -13,7 +13,7 @@
 #define INT_DELIVERY_MODE (genapic.int_delivery_mode)
 #define INT_DEST_MODE (genapic.int_dest_mode)
 #define TARGET_CPUS ((const typeof(cpu_online_map) *)&cpu_online_map)
-#define init_apic_ldr (genapic.init_apic_ldr)
+#define init_apic_ldr() alternative_vcall(genapic.init_apic_ldr)
 #define cpu_mask_to_apicid(mask) ({ \
 	/* \
 	 * There are a number of places where the address of a local variable \