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x86/amd_node: Avoid divide by zero on virtualized systems

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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: 0a4b61d9c2 ("x86/amd_node: Fix AMD root device caching")
Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Yazen Ghannam <yazen.ghannam@amd.com>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260825214805.39148-2-jason.andryuk@amd.com
This commit is contained in:
Jason Andryuk 2026-08-25 17:48:02 -04:00 committed by Borislav Petkov (AMD)
parent cee9395acd
commit 72bd92bd81
1 changed files with 5 additions and 0 deletions
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5
arch/x86/kernel/amd_node.c
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@ -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;