[tip: x86/urgent] x86/amd_node: Avoid divide by zero on virtualized systems

[tip-bot2 for Jason Andryuk]

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     72bd92bd8190d7869ecb462649ca40f297822a33
Gitweb:        https://git.kernel.org/tip/72bd92bd8190d7869ecb462649ca40f297822a33
Author:        Jason Andryuk <jason.andryuk@xxxxxxx>
AuthorDate:    Tue, 25 Aug 2026 17:48:02 -04:00
Committer:     Borislav Petkov (AMD) <bp@xxxxxxxxx>
CommitterDate: Sun, 30 Aug 2026 18:25:42 -07:00

x86/amd_node: Avoid divide by zero on virtualized systems

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)

because the underlying topology code on Xen ends up making num_nodes
2 and num_roots 1 and the integer division result is 0.

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

Set roots_per_node to 1 in this case. Print a firmware bug when this is
performed for non-virtualized systems.

  [ bp: Massage commit message. ]

Fixes: 0a4b61d9c2e4 ("x86/amd_node: Fix AMD root device caching")
Suggested-by: Borislav Petkov <bp@xxxxxxxxx>
Signed-off-by: Jason Andryuk <jason.andryuk@xxxxxxx>
Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Reviewed-by: Yazen Ghannam <yazen.ghannam@xxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: https://patch.msgid.link/20260825214805.39148-2-jason.andryuk@xxxxxxx
---
 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 0be0172..408b9fd 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.\n");
+		roots_per_node = 1;
+	}
 
 	count = 0;
 	node = 0;