Re: [PATCH v2 1/2] x86/amd_node: Avoid divide by zero on virtualized systems

From: Jason Andryuk

Date: Tue Aug 25 2026 - 00:23:05 EST

On 2026-08-24 17:04, Yazen Ghannam wrote:
On Mon, Aug 24, 2026 at 04:05:40PM -0400, Jason Andryuk wrote:
Hi,

On 2026-08-14 17:42, Jason Andryuk wrote:
On a virtualized system, the number of nodes does not have a
relationship to the number of roots. A Xen PVH dom0 can calculate
roots_per_node as 0, which crashes with a divide by zero in:

if (count++ % roots_per_node)

The issue is seen with Xen, but it could affect other systems.

Change roots_per_node == 0 to 1 to allow the system to boot. Print a
warning when this is performed for non-virtualized systems.

Fixes: 0a4b61d9c2e4 ("x86/amd_node: Fix AMD root device caching")
Cc: stable@xxxxxxxxxxxxxxx
Suggested-by: Borislav Petkov <bp@xxxxxxxxx>
Signed-off-by: Jason Andryuk <jason.andryuk@xxxxxxx>

Any feedback here?

Thanks,
Jason

---
v2:
Print as a warning instead of error
Use "!" instead of "== 0".

X86_FEATURE_XENPV is only for PV, but this is observed with a PVH dom0.
---
arch/x86/kernel/amd_node.c | 5 +++++
1 file changed, 5 insertions(+)

diff --git a/arch/x86/kernel/amd_node.c b/arch/x86/kernel/amd_node.c
index 0be01725a2a4..c3e214925d9c 100644
--- a/arch/x86/kernel/amd_node.c
+++ b/arch/x86/kernel/amd_node.c
@@ -287,6 +287,11 @@ static int __init amd_smn_init(void)
return -ENOMEM;
roots_per_node = num_roots / num_nodes;
+ if (!roots_per_node) {
+ if (!cpu_feature_enabled(X86_FEATURE_HYPERVISOR))
+ pr_warn(FW_BUG "Error detecting roots per node.");
+ roots_per_node = 1;
+ }

Minor nit: add a newline after the closing brace.

There is an existing newline that is preserved here in the original posting. It seems when I replied in Thunderbird the context here was corrupted.

count = 0;
node = 0;


Overall, looks good to me.

Reviewed-by: Yazen Ghannam <yazen.ghannam@xxxxxxx>
Thank you.

Jason