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 <t...@linutronix.de>
Cc: x...@kernel.org
Cc: xen-devel@lists.xenproject.org
Cc: sta...@vger.kernel.org # 6.9+
Signed-off-by: Petr Vaněk <arka...@atlas.cz>
---
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
