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Re: [PATCH v1 0/2] x86/amd_node: harden amd_smn_init() against Xen dom0 topology
- To: Penny Zheng <penny.zheng@xxxxxxx>, x86@xxxxxxxxxx
- From: Mario Limonciello <mario.limonciello@xxxxxxx>
- Date: Wed, 6 May 2026 12:17:24 -0500
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- Cc: ray.huang@xxxxxxx, Jason.Andryuk@xxxxxxx, stefano.stabellini@xxxxxxx, Yazen Ghannam <yazen.ghannam@xxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, Borislav Petkov <bp@xxxxxxxxx>, Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx
- Delivery-date: Wed, 06 May 2026 17:17:46 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 5/6/26 00:55, Penny Zheng wrote:
While booting a recent linux-next kernel as a Xen PVH dom0 on x86, the kernel
oopses very early during fs_initcall:
Oops: divide error: 0000 [#1] SMP NOPTI
RIP: 0010:amd_smn_init+0x188/0x2e0
Followed: on a kernel that survives the divide, it will fail by a NULL pointer
dereference from the first SMN consumer (amd_pmc_probe -> amd_smn_read).
So to confirm - does amd_pmc_probe work properly with this series now?
Root cause
==========
To prevent each dom0 vCPU from looking like an SMT sibling of another
vCPU, Xen synthesizes guest x2APIC IDs as vcpu_index * 2. This spacing every
vCPU's APIC ID by 2 can push the synthesized IDs past the package-field
boundary. Linux then infers more "packages" and therefore more AMD
nodes via amd_num_nodes() than the platform actually has, while the
PCI-side host-bridge scan correctly reports the number of root complex.
The fixes are tested on Xen 4.20 PVH dom0 on AMD Zen (16 vCPUs) on top of
linux-next/master (next-20260505).
Penny Zheng (2):
x86/amd_node: avoid divide-by-zero in amd_smn_init() under Xen dom0
x86/amd_node: reject SMN access when amd_smn_init() did not complete
arch/x86/kernel/amd_node.c | 23 ++++++++++++++++++++---
1 file changed, 20 insertions(+), 3 deletions(-)
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