Re: [PATCH v4 1/6] x86/resctrl: Parse ACPI ERDT table and map RMDD domains by L3 cache ID

[Reinette Chatre]

Hi Tony,

On 6/24/26 4:58 PM, Luck, Tony wrote:
>> I do not see the need for an xarray. As new domains are created during CPU online their
>> initialization can just point to the needed data, no?
> 
> Reinette,
> 
> Historical note. The xarray was added during early development on a simulator where the L3
> domain numbers were wildly sparse. That turned out to be an artifact of the simulator
> CPUID and BIOS interactions. On real h/w the L3 domains show up as 0, 1, 2, 3, ... 

resctrl dynamically allocates monitoring domains as the CPUs come online and the monitoring
domains are assigned the domain ID discovered from the system. This should not be impacted by
sparse domain numbers and works without an xarray, no? What am I missing?

> But the xarray does still serve some purpose. Without it you’d need two passes through the
> ACPI ERDT structure. One to count how many RMDD substructures are present to allocate
> space, then second pass to parse things.
The data structures containing the domain data discovered from ERDT could just be dynamically
allocated with a single pass as the domains are discovered, similar to how rdt_resource::mon_domains
is populated, no? The difference is that the "ERDT domains" are discovered during initialization.

I do not see a need for a separate xarray indexed by domain ID during "runtime" since we already
have the architecture specific domain structures. struct rdt_l3_mon_domain could just point to
its associated ERDT domain when it (struct rdt_l3_mon_domain) is initialized, no?

You could use an xarray during initial RMDD parsing but from what I understand its index
is of no use since it was established early in this discussion that RMDD domain ID cannot be
assumed to be the same as enumerated via CPUID leaf 0x4. So even if you use an xarray the
CPU online handler needs to iterate over the xarray instead of indexing directly into it
to find CPU being onlined and through that determine the "RMDD domain ID" associated with the
CPUID leaf 0x4 cache ID of CPU being onlined. What is the benefit of an xarray when never
indexing into it?

Reinette