[PATCH] x86/topology: Tolerate lack of APIC when booting as Xen domU

[Ariadne Conill]

Xen domU instances do not boot on x86 with ACPI enabled, so the entire
ACPI subsystem is ultimately disabled.  This causes acpi_mps_check()
to trigger a warning that the ACPI MPS table is not present, which then
disables APIC support on domU, breaking the CPU topology detection for
all vCPUs other than the boot vCPU.

Fixes: 7c0edad3643f ("x86/cpu/topology: Rework possible CPU management")
Signed-off-by: Ariadne Conill <ariadne@ariadne.space>
Cc: x86@xxxxxxxxxx
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
---
 arch/x86/kernel/cpu/topology.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/topology.c b/arch/x86/kernel/cpu/topology.c
index 621a151ccf7d..38fa5ed816d6 100644
--- a/arch/x86/kernel/cpu/topology.c
+++ b/arch/x86/kernel/cpu/topology.c
@@ -429,7 +429,7 @@ void __init topology_apply_cmdline_limits_early(void)
 	unsigned int possible = nr_cpu_ids;
 
 	/* 'maxcpus=0' 'nosmp' 'nolapic' 'disableapic' 'noapic' */
-	if (!setup_max_cpus || ioapic_is_disabled || apic_is_disabled)
+	if (!setup_max_cpus || ioapic_is_disabled || (apic_is_disabled && !xen_pv_domain()))
 		possible = 1;
 
 	/* 'possible_cpus=N' */
-- 
2.39.2