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

[Mario Limonciello]

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(-)