Commit 8b37357a authored by Petr Vaněk's avatar Petr Vaněk Committed by Thomas Gleixner
Browse files

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 7c0edad3
("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: 7c0edad3 ("x86/cpu/topology: Rework possible CPU management")
Signed-off-by: default avatarPetr Vaněk <arkamar@atlas.cz>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250407132445.6732-2-arkamar@atlas.cz
parent 0af2f6be

arch/x86/kernel/acpi/boot.c

+11 −0
Original line number Diff line number Diff line
@@ -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;