Re: [XEN RFC PATCH 37/40] xen: introduce an arch helper to do NUMA init failed fallback

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.08.2021 05:45, Wei Chen wrote:
>> From: Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of Wei
>> Chen
>> Sent: 2021年8月28日 11:09
>>
>>> From: Julien Grall <julien@xxxxxxx>
>>> Sent: 2021年8月27日 22:30
>>>
>>>> --- a/xen/arch/arm/numa.c
>>>> +++ b/xen/arch/arm/numa.c
>>>> @@ -140,3 +140,16 @@ int __init arch_meminfo_get_ram_bank_range(int
>> bank,
>>>>
>>>>    return 0;
>>>>   }
>>>> +
>>>> +void __init arch_numa_init_failed_fallback(void)
>>>> +{
>>>> +    int i, j;
>>>> +
>>>> +    /* Reset all node distance to remote_distance */
>>>> +    for ( i = 0; i < MAX_NUMNODES; i++ ) {
>>>> +        for ( j = 0; j < MAX_NUMNODES; j++ ) {
>>>> +            numa_set_distance(i, j,
>>>> +                (i == j) ? NUMA_LOCAL_DISTANCE :
>> NUMA_REMOTE_DISTANCE);
>>>> +        }
>>>> +    }
>>>> +}
>>>
>>> ... this implementation looks fairly generic. So can you explain why we
>>> need it on Arm but not x86?
>>>
>>
>> This implementation is DT only, for x86, it's using acpi_slit.
>> For now, I am not quit sure ACPI need to do fallback or not.
>> Or say in another way, I don't know how to implement the fallback
>> for ACPI. I planned to solve it in Arm ACPI version NUMA, so I left
>> an empty helper for x86.
>>
>> @Jan Beulich Could you give me some suggestion about x86 fallback?
>>
>>
> 
> I have a quick look into Linux. When Arch do numa init failed,
> the numa_free_distance will be invoked to revert numa_distance.

Does this matter in the first place? Don't we fall back to single
node mode, in which case the sole entry of the distance table
will say "local" anyway?

Jan