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

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

On 2026-08-25 00:00, Borislav Petkov wrote:
> On Fri, Aug 14, 2026 at 05:42:52PM -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>
>> Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
>> ---
>> 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;
>> +	}
>>   
>>   	count = 0;
>>   	node = 0;
>> -- 
> 
> https://sashiko.dev/#/patchset/20260814214255.83127-1-jason.andryuk%40amd.com
> 
> And this one specifically (the other two are being addressed):
> 
> "Are we missing a newline character at the end of this warning message?
> Without a trailing \n in the format string, subsequent kernel messages might
> be appended to the same line, resulting in malformed dmesg output."

Yes, thanks.  I'll fix.

Regards,
Jason