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

[Borislav Petkov]

On Fri, Aug 14, 2026 at 05:42:52PM -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>
> Signed-off-by: Jason Andryuk <jason.andryuk@xxxxxxx>
> ---
> 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;
> +	}
>  
>  	count = 0;
>  	node = 0;
> -- 

https://sashiko.dev/#/patchset/20260814214255.83127-1-jason.andryuk%40amd.com

And this one specifically (the other two are being addressed):

"Are we missing a newline character at the end of this warning message?
Without a trailing \n in the format string, subsequent kernel messages might
be appended to the same line, resulting in malformed dmesg output."

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette