Merge tag 'x86-urgent-2025-04-10' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull misc x86 fixes from Ingo Molnar:

 - Fix CPU topology related regression that limited Xen PV guests to a
   single CPU

 - Fix ancient e820__register_nosave_regions() bugs that were causing
   problems with kexec's artificial memory maps

 - Fix an S4 hibernation crash caused by two missing ENDBR's that were
   mistakenly removed in a recent commit

 - Fix a resctrl serialization bug

 - Fix early_printk documentation and comments

 - Fix RSB bugs, combined with preparatory updates to better match the
   code to vendor recommendations.

 - Add RSB mitigation document

 - Fix/update documentation

 - Fix the erratum_1386_microcode[] table to be NULL terminated

* tag 'x86-urgent-2025-04-10' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/ibt: Fix hibernate
  x86/cpu: Avoid running off the end of an AMD erratum table
  Documentation/x86: Zap the subsection letters
  Documentation/x86: Update the naming of CPU features for /proc/cpuinfo
  x86/bugs: Add RSB mitigation document
  x86/bugs: Don't fill RSB on context switch with eIBRS
  x86/bugs: Don't fill RSB on VMEXIT with eIBRS+retpoline
  x86/bugs: Fix RSB clearing in indirect_branch_prediction_barrier()
  x86/bugs: Use SBPB in write_ibpb() if applicable
  x86/bugs: Rename entry_ibpb() to write_ibpb()
  x86/early_printk: Use 'mmio32' for consistency, fix comments
  x86/resctrl: Fix rdtgroup_mkdir()'s unlocked use of kernfs_node::name
  x86/e820: Fix handling of subpage regions when calculating nosave ranges in e820__register_nosave_regions()
  x86/acpi: Don't limit CPUs to 1 for Xen PV guests due to disabled ACPI

1 files changed, 11 insertions, 0 deletions

diff --git a/arch/x86/kernel/acpi/boot.c b/arch/x86/kernel/acpi/boot.c
index dae6a73be40e..9fa321a95eb3 100644
--- a/arch/x86/kernel/acpi/boot.c
+++ b/arch/x86/kernel/acpi/boot.c
@@ -23,6 +23,8 @@
 #include <linux/serial_core.h>
 #include <linux/pgtable.h>
 
+#include <xen/xen.h>
+
 #include <asm/e820/api.h>
 #include <asm/irqdomain.h>
 #include <asm/pci_x86.h>
@@ -1729,6 +1731,15 @@ int __init acpi_mps_check(void)
 {
 #if defined(CONFIG_X86_LOCAL_APIC) && !defined(CONFIG_X86_MPPARSE)
 /* mptable code is not built-in*/
+
+	/*
+	 * Xen disables ACPI in PV DomU guests but it still emulates APIC and
+	 * supports SMP. Returning early here ensures that APIC is not disabled
+	 * unnecessarily and the guest is not limited to a single vCPU.
+	 */
+	if (xen_pv_domain() && !xen_initial_domain())
+		return 0;
+
 	if (acpi_disabled || acpi_noirq) {
 		pr_warn("MPS support code is not built-in, using acpi=off or acpi=noirq or pci=noacpi may have problem\n");
 		return 1;