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

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, the local 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.

Update the 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")
Signed-off-by: Petr Vaněk <arkamar@atlas.cz>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250407132445.6732-2-arkamar@atlas.cz

diff --git a/arch/x86/kernel/acpi/boot.c b/arch/x86/kernel/acpi/boot.c
index dae6a73be40e1cce5fcab45d66607ddb027d2a2b..9fa321a95eb33faf8a4b7c6f45918013c8cd3fb8 100644 (file)
--- 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;