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 <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/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
