Re: [PATCH v3 1/2] x86/amd_node: Avoid divide by zero on virtualized systems
From: Jason Andryuk
Date: Wed Aug 26 2026 - 08:16:04 EST
On 2026-08-26 00:45, Borislav Petkov wrote:
On Tue, Aug 25, 2026 at 05:48:02PM -0400, Jason Andryuk wrote:num_roots is 1, but num_nodes is 2. Integer division gives 0 for roots_per_node.
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>
Reviewed-by: Yazen Ghannam <yazen.ghannam@xxxxxxx>
Signed-off-by: Jason Andryuk <jason.andryuk@xxxxxxx>
---
v3:
Add newline to warning.
R-b: Yazen
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..408b9fd48349 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;
Hold on - the only way this is fixing something on Xen - and you say that
roots_per_node becomes 0 - which means that num_roots is 0.
But a couple of lines above, we have
if (!num_roots)
return -ENODEV;
So we would've exited already.
num_nodes can't be 0 because you'd explode even with this patch.
What am I missing?
Regards,
Jason