x86: arrange for ENDBR zapping from <vendor>_ctxt_switch_masking()

While altcall is already used for them, the functions want announcing in
.init.rodata.cf_clobber, even if the resulting static variables aren't
otherwise used.

While doing this also move ctxt_switch_masking to .data.ro_after_init.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
(cherry picked from commit 044168fa3a)

xen/arch/x86/cpu/amd.c

@@ -258,6 +258,11 @@ static void cf_check amd_ctxt_switch_masking(const struct vcpu *next)
 #undef LAZY
 }
 
+#ifdef CONFIG_XEN_IBT /* Announce the function to ENDBR clobbering logic. */
+static const typeof(ctxt_switch_masking) __initconst_cf_clobber __used csm =
+    amd_ctxt_switch_masking;
+#endif
+
 /*
  * Mask the features and extended features returned by CPUID.  Parameters are
  * set from the boot line via two methods:

xen/arch/x86/cpu/common.c

@@ -121,7 +121,7 @@ static const struct cpu_dev default_cpu = {
 static const struct cpu_dev *this_cpu = &default_cpu;
 
 static DEFINE_PER_CPU(uint64_t, msr_misc_features);
-void (* __read_mostly ctxt_switch_masking)(const struct vcpu *next);
+void (* __ro_after_init ctxt_switch_masking)(const struct vcpu *next);
 
 bool __init probe_cpuid_faulting(void)
 {

xen/arch/x86/cpu/intel.c

@@ -220,6 +220,11 @@ static void cf_check intel_ctxt_switch_masking(const struct vcpu *next)
 #undef LAZY
 }
 
+#ifdef CONFIG_XEN_IBT /* Announce the function to ENDBR clobbering logic. */
+static const typeof(ctxt_switch_masking) __initconst_cf_clobber __used csm =
+    intel_ctxt_switch_masking;
+#endif
+
 /*
  * opt_cpuid_mask_ecx/edx: cpuid.1[ecx, edx] feature mask.
  * For example, E8400[Intel Core 2 Duo Processor series] ecx = 0x0008E3FD,