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

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

On 2026-08-24 17:04, Yazen Ghannam wrote:
> On Mon, Aug 24, 2026 at 04:05:40PM -0400, Jason Andryuk wrote:
>> Hi,
>>
>> On 2026-08-14 17:42, 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@vger.kernel.org
>>> Suggested-by: Borislav Petkov <bp@alien8.de>
>>> Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
>>
>> Any feedback here?
>>
>> Thanks,
>> Jason
>>
>>> ---
>>> 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;
>>> +	}
> 
> Minor nit: add a newline after the closing brace.

There is an existing newline that is preserved here in the original 
posting.  It seems when I replied in Thunderbird the context here was 
corrupted.

>>>    	count = 0;
>>>    	node = 0;
>>
> 
> Overall, looks good to me.
> 
> Reviewed-by: Yazen Ghannam <yazen.ghannam@amd.com>
Thank you.

Jason