x86/APIC: avoid iommu_supports_x2apic() on error path

The value it returns may change from true to false in case
iommu_enable_x2apic() fails and, as a side effect, clears iommu_intremap
(as can happen at least on AMD). Latch the return value from the first
invocation to replace the second one.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>

xen/arch/x86/apic.c

@@ -865,6 +865,7 @@ void x2apic_ap_setup(void)
 void __init x2apic_bsp_setup(void)
 {
     struct IO_APIC_route_entry **ioapic_entries = NULL;
+    bool iommu_x2apic;
     const char *orig_name;
 
     if ( !cpu_has_x2apic )
@@ -880,7 +881,8 @@ void __init x2apic_bsp_setup(void)
         printk("x2APIC: Already enabled by BIOS: Ignoring cmdline disable.\n");
     }
 
-    if ( iommu_supports_x2apic() )
+    iommu_x2apic = iommu_supports_x2apic();
+    if ( iommu_x2apic )
     {
         if ( (ioapic_entries = alloc_ioapic_entries()) == NULL )
         {
@@ -933,8 +935,11 @@ void __init x2apic_bsp_setup(void)
         printk("Switched to APIC driver %s\n", genapic.name);
 
 restore_out:
-    /* iommu_x2apic_enabled cannot be used here in the error case. */
-    if ( iommu_supports_x2apic() )
+    /*
+     * iommu_x2apic_enabled and iommu_supports_x2apic() cannot be used here
+     * in the error case.
+     */
+    if ( iommu_x2apic )
     {
         /*
          * NB: do not use raw mode when restoring entries if the iommu has