linux-kernel - [PATCH 0/1] x86/topology: Fix regression limiting Xen PV DomU CPUs to 1

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Message-ID: <20250405181650.22827-1-arkamar@atlas.cz>
Date: Sat,  5 Apr 2025 20:16:49 +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>
Subject: [PATCH 0/1] x86/topology: Fix regression limiting Xen PV DomU CPUs to 1

Hi,

I have discovered a regression in recent kernels that causes Xen PV DomU
guests to be limited to a single vCPU, despite multiple vCPUs being
configured. This issue does no occur in version 6.6.

After bisecting, I identified the regression was introduce in kernel 6.9
in a commit that limits the number of possible CPUs to 1 when APIC is
disabled [1]. However, Xen PV guests always disable APIC, leading to
incorrect CPU limitation.

Following patch resolves this issue by skipping apic_is_disabled check
for Xen PV guests.

I believe it makes sense to backport this fix to stable versions, so I'm
Ccing stable@...r.kernel.org (as per [2]). Note that there is a minor
conflict in both applicable versions (6.12 and 6.13). Please, let me
know if a different approach would be preferred.

[1] https://lore.kernel.org/r/20240213210252.290098853@linutronix.de
[2] https://docs.kernel.org/process/stable-kernel-rules.html#option-1

Thanks,
Petr

Petr Vaněk (1):
  x86/cpu/topology: Don't limit CPUs to 1 for Xen PV guests due to
    disabled APIC

 arch/x86/kernel/cpu/topology.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

-- 
2.48.1