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

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Message-Id: <20240903193606.49830-1-ariadne@ariadne.space>
Date: Tue,  3 Sep 2024 12:36:06 -0700
From: Ariadne Conill <ariadne@...adne.space>
To: linux-kernel@...r.kernel.org
Cc: Ariadne Conill <ariadne@...adne.space>,
	x86@...nel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	stable@...r.kernel.org
Subject: [PATCH] x86/topology: Tolerate lack of APIC when booting as Xen domU

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@...adne.space>
Cc: x86@...nel.org
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: linux-kernel@...r.kernel.org
Cc: stable@...r.kernel.org
---
 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