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

[Jason Andryuk <jason.andryuk@amd.com>]

On 2026-08-11 04:21, David Laight wrote:
> On Mon, 10 Aug 2026 10:47:06 -0400
> Yazen Ghannam <yazen.ghannam@amd.com> 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: 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);
> 
> I'd not add the max(), just;
> 	if (!roots_per_node) {
> 		roots_per_node = 1;
> 		if (cpu_feature_enabled(X86_FEATURE_HYPERVISOR)
> 			pr_warn("Expected at least 1 root per AMD node.\n");
> 	}

I prefer this because it puts the special handing inside the check. 
David, you dropped ! from X86_FEATURE_HYPERVISOR - did you mean that? 
The intent was to only warn if real hardware doesn't match the expectation.

Really, I just want to get this fixed.

Thanks,
Jason