x86/iommu: setup MMCFG ahead of IOMMU

Otherwise the PCI accesses to segments different than the first one done by
the IOMMU initialization code would silently fail by returning all ones.

Introduce a new helper, called pci_setup(), and move both the creation of
PCI segment 0 internal data structures, plus the parsing of ACPI MMCFG
table to it.  This moves acpi_mmcfg_init() slightly earlier from
acpi_boot_init() into pci_setup().

Note that further work will be needed to support systems where access
methods to segments different than 0 is not discoverable by Xen.

Fixes: 3950f2485b ('x86/x2APIC: defer probe until after IOMMU ACPI table parsing')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
master commit: c292772b49
master date: 2025-08-21 11:57:25 +0200

xen/arch/x86/acpi/boot.c

@@ -748,8 +748,6 @@ int __init acpi_boot_init(void)
 
 	acpi_table_parse(ACPI_SIG_HPET, acpi_parse_hpet);
 
-	acpi_mmcfg_init();
-
 	erst_init();
 
 	acpi_hest_init();

xen/arch/x86/setup.c

@@ -1802,6 +1802,12 @@ void asmlinkage __init noreturn __start_xen(unsigned long mbi_p)
      */
     setup_system_domains();
 
+    /*
+     * Initialize PCI (create segment 0, setup MMCFG access) ahead of IOMMU
+     * setup, as devices in segment > 0 must also be discoverable.
+     */
+    acpi_mmcfg_init();
+
     /*
      * IOMMU-related ACPI table parsing has to happen before APIC probing, for
      * check_x2apic_preenabled() to be able to observe respective findings, in

xen/arch/x86/x86_64/mmconfig-shared.c

@@ -403,6 +403,9 @@ void __init acpi_mmcfg_init(void)
 
     pci_segments_init();
 
+    if ( acpi_disabled )
+        return;
+
     /* MMCONFIG disabled */
     if ((pci_probe & PCI_PROBE_MMCONF) == 0)
         return;