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

[Yazen Ghannam <yazen.ghannam@amd.com>]

On Thu, Aug 06, 2026 at 12:01:57PM -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)
> 
> On a virtualized system, default the value to 1.  The issue is seen with
> Xen, but it could affect other systems.
> 
> Fixes: 0a4b61d9c2e4 ("x86/amd_node: Fix AMD root device caching")
> Cc: stable@vger.kernel.org
> Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>

I agree with the idea with some minor feedback below.

> ---
> 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 ea553267e5fa..c5025e5291b6 100644
> --- a/arch/x86/kernel/amd_node.c
> +++ b/arch/x86/kernel/amd_node.c
> @@ -286,6 +286,11 @@ static int __init amd_smn_init(void)
>  		return -ENOMEM;
>  
>  	roots_per_node = num_roots / num_nodes;
> +	if (roots_per_node == 0) {

Can be '!roots_per_node'.

> +		if (!cpu_feature_enabled(X86_FEATURE_HYPERVISOR))
> +			pr_err(FW_BUG "Error detecting roots per node.\n");
> +		roots_per_node = 1;
> +	}

I don't think this is necessarily a Firmware bug.

Likewise, it can be a 'warning' rather than 'error'. I interpret 'error'
as something known to be incorrect. This was my position before on this
topic. But the various contrary reports changed my mind (even if they
come from virtualization).

Furthermore, I think 'warning' is more appropriate. It says "We found
something unexpected. We're letting you (the user) know about it. And
we'll mitigate it to avoid an error."

Anyways, I just wanted to write out some thoughts since this topic has
come up a few times.

Possible rework for the above change:

	if (!roots_per_node && !cpu_feature_enabled(X86_FEATURE_HYPERVISOR)
		pr_warn("Expected at least 1 root per AMD node.\n");

	roots_per_node = max(roots_per_node, 1);

In any case, we should just go with a simple fix for the virt cases.

I think we could even do away with caching the 'root' devices. But
that'll be another rework/cleanup.

Thanks,
Yazen