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

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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 7c0edad364
("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: 7c0edad364 ("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
This commit is contained in:
Petr Vaněk 2025-04-07 15:24:27 +02:00 committed by Thomas Gleixner
parent 0af2f6be1b
commit 8b37357a78
1 changed files with 11 additions and 0 deletions
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11
arch/x86/kernel/acpi/boot.c
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@ -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;