Re: [PATCH v1 0/2] x86/amd_node: harden amd_smn_init() against Xen dom0 topology

[Borislav Petkov <bp@xxxxxxxxx>]

On Wed, May 06, 2026 at 01:55:17PM +0800, 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).
> 
> 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.

Does that work?

diff --git a/arch/x86/kernel/amd_node.c b/arch/x86/kernel/amd_node.c
index 0be01725a2a4..0f1ad1883883 100644
--- a/arch/x86/kernel/amd_node.c
+++ b/arch/x86/kernel/amd_node.c
@@ -249,6 +249,9 @@ static int __init amd_smn_init(void)
        u16 count, num_roots, roots_per_node, node, num_nodes;
        struct pci_dev *root;
 
+       if (cpu_feature_enabled(X86_FEATURE_HYPERVISOR))
+               return 0;
+
        if (!cpu_feature_enabled(X86_FEATURE_ZEN))
                return 0;
 

Totally untested ofc.

-- 
Regards/Gruss,
    Boris.

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