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

On 2026-08-10 10:47, Yazen Ghannam wrote:
> 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: [email protected]
>> Signed-off-by: Jason Andryuk <[email protected]>
> 
> 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'.

Sure.
>> +		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."

Makes sense to me.  Boris suggested error in 
https://lore.kernel.org/lkml/20260624192530.GGajwvKuhxbnHe0s42@fat_crate.local/

(I probably should have used Suggested-by.)

> 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'm okay with this.  I'll wait to see if anyone chimes in further.  If 
not, I'll go with your approach and Suggested-by.

Thanks,
Jason