Re: [XEN RFC PATCH 16/40] xen/arm: Create a fake NUMA node to use common code

[Jan Beulich <jbeulich@xxxxxxxx>]

On 27.08.2021 01:10, Stefano Stabellini wrote:
> On Wed, 11 Aug 2021, Wei Chen wrote:
>> @@ -29,3 +31,54 @@ void numa_set_node(int cpu, nodeid_t nid)
>>  
>>      cpu_to_node[cpu] = nid;
>>  }
>> +
>> +void __init numa_init(bool acpi_off)
>> +{
>> +    uint32_t idx;
>> +    paddr_t ram_start = ~0;
>> +    paddr_t ram_size = 0;
>> +    paddr_t ram_end = 0;
>> +
>> +    printk(XENLOG_WARNING
>> +        "NUMA has not been supported yet, NUMA off!\n");
> 
> NIT: please align
> 
> 
>> +    /* Arm NUMA has not been implemented until this patch */
> 
> "Arm NUMA is not implemented yet"
> 
> 
>> +    numa_off = true;
>> +
>> +    /*
>> +     * Set all cpu_to_node mapping to 0, this will make cpu_to_node
>> +     * function return 0 as previous fake cpu_to_node API.
>> +     */
>> +    for ( idx = 0; idx < NR_CPUS; idx++ )
>> +        cpu_to_node[idx] = 0;
>> +
>> +    /*
>> +     * Make node_to_cpumask, node_spanned_pages and node_start_pfn
>> +     * return as previous fake APIs.
>> +     */
>> +    for ( idx = 0; idx < MAX_NUMNODES; idx++ ) {
>> +        node_to_cpumask[idx] = cpu_online_map;
>> +        node_spanned_pages(idx) = (max_page - mfn_x(first_valid_mfn));
>> +        node_start_pfn(idx) = (mfn_x(first_valid_mfn));
>> +    }
> 
> I just want to note that this works because MAX_NUMNODES is 1. If
> MAX_NUMNODES was > 1 then it would be wrong to set node_to_cpumask,
> node_spanned_pages and node_start_pfn for all nodes to the same values.
> 
> It might be worth writing something about it in the in-code comment.

Plus perhaps BUILD_BUG_ON(MAX_NUMNODES != 1), so the issue is actually
noticed at build time once the constant gets changed?

Jan