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@linutronix.de Cc: x86@kernel.org Cc: xen-devel@lists.xenproject.org Cc: stable@vger.kernel.org # 6.9+ Signed-off-by: Petr Vaněk arkamar@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;