linux-kernel - [PATCH 1/1] x86/cpu/topology: Don't limit CPUs to 1 for Xen PV guests due to disabled APIC

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Message-ID: <20250405181650.22827-2-arkamar@atlas.cz>
Date: Sat,  5 Apr 2025 20:16:50 +0200
From: Petr Vaněk <arkamar@...as.cz>
To: linux-kernel@...r.kernel.org
Cc: Thomas Gleixner <tglx@...utronix.de>,
	x86@...nel.org,
	xen-devel@...ts.xenproject.org,
	Petr Vaněk <arkamar@...as.cz>,
	stable@...r.kernel.org
Subject: [PATCH 1/1] x86/cpu/topology: Don't limit CPUs to 1 for Xen PV guests due to disabled APIC

Xen PV guests in DomU have APIC disabled by design, which causes
topology_apply_cmdline_limits_early() to limit the number of possible
CPUs to 1, regardless of the configured number of vCPUs.

This is a regression introduced in version 6.9 in commit 7c0edad3643f
("x86/cpu/topology: Rework possible CPU management") which added an
early check that limits CPUs if apic_is_disabled, without accounting for
the fact that Xen PV guests always disable APIC even when SMP is
supported.

This patch fixes the issue by skipping the apic_is_disabled check for
Xen PV guests, allowing them to boot with the full set of configured vCPUs.

Fixes: 7c0edad3643f ("x86/cpu/topology: Rework possible CPU management")
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: x86@...nel.org
Cc: xen-devel@...ts.xenproject.org
Cc: stable@...r.kernel.org # 6.9+
Signed-off-by: Petr Vaněk <arkamar@...as.cz>
---
 arch/x86/kernel/cpu/topology.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/topology.c b/arch/x86/kernel/cpu/topology.c
index 01456236a6dd..10aa7f471ec9 100644
--- a/arch/x86/kernel/cpu/topology.c
+++ b/arch/x86/kernel/cpu/topology.c
@@ -428,8 +428,13 @@ void __init topology_apply_cmdline_limits_early(void)
 {
 	unsigned int possible = nr_cpu_ids;
 
-	/* 'maxcpus=0' 'nosmp' 'nolapic' */
-	if (!setup_max_cpus || apic_is_disabled)
+	/* 'maxcpus=0' 'nosmp' 'nolapic'
+	 *
+	 * The apic_is_disabled check is ignored for Xen PV domains because Xen
+	 * disables ACPI in unprivileged PV DomU guests, which would otherwise limit
+	 * CPUs to 1, even if multiple vCPUs were configured.
+	 */
+	if (!setup_max_cpus || (!xen_pv_domain() && apic_is_disabled))
 		possible = 1;
 
 	/* 'possible_cpus=N' */
-- 
2.48.1