Re: [PATCH v3 1/2] x86/amd_node: Avoid divide by zero on virtualized systems

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

On 2026-08-26 00:45, Borislav Petkov wrote:
> On Tue, Aug 25, 2026 at 05:48:02PM -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@vger.kernel.org
>> Suggested-by: Borislav Petkov <bp@alien8.de>
>> Reviewed-by: Yazen Ghannam <yazen.ghannam@amd.com>
>> Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
>> ---
>> v3:
>> Add newline to warning.
>> R-b: Yazen
>>
>> 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..408b9fd48349 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;
> 
> Hold on - the only way this is fixing something on Xen - and you say that
> roots_per_node becomes 0 - which means that num_roots is 0.
> 
> But a couple of lines above, we have
> 
>          if (!num_roots)
>                  return -ENODEV;
> 
> So we would've exited already.
> 
> num_nodes can't be 0 because you'd explode even with this patch.
> 
> What am I missing?
num_roots is 1, but num_nodes is 2.  Integer division gives 0 for 
roots_per_node.

Regards,
Jason