[PATCH v2 1/1] x86/acpi: Don't limit CPUs to 1 for Xen PV guests due to disabled ACPI

[Petr Vaněk]

Xen disables ACPI for PV guests in DomU, which causes acpi_mps_check()
to return 1 when CONFIG_X86_MPPARSE is not set. As a result, APIC is
disabled and the guest is later limited to a single vCPU, despite being
configured with more.

This regression was introduced in version 6.9 in commit 7c0edad3643f
("x86/cpu/topology: Rework possible CPU management"), which added an
early check that limits CPUs to 1 if apic_is_disabled.

This patch updates acpi_mps_check() logic to return 0 early when running
as a Xen PV guest in DomU, preventing APIC from being disabled in this
specific case and restoring correct multi-vCPU behaviour.

Fixes: 7c0edad3643f ("x86/cpu/topology: Rework possible CPU management")
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx # 6.9+
Signed-off-by: Petr Vaněk <arkamar@xxxxxxxx>
---
 arch/x86/kernel/acpi/boot.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

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;
-- 
2.48.1