Re: [PATCH v3 1/2] x86/amd_node: Avoid divide by zero on virtualized systems

[Borislav Petkov]

On Tue, Aug 25, 2026 at 05:48:02PM -0400, 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>
> 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?

Thx.

-- 
Regards/Gruss,
    Boris.

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