Re: [PATCH v6 2/9] x86/resctrl: Require 64-bit x86 for resctrl support

[Reinette Chatre]

Hi Tony,

On 8/20/26 8:20 AM, Luck, Tony wrote:
>> Reviewed at:
>> https://lore.kernel.org/lkml/d97deaa1-f42c-4fce-8ca8-5ccf8d822cd2@xxxxxxxxx/
> 
> Reinette,
> 
> I've written up this draft for next series. Are you looking for something like this?
> 
>     x86/resctrl: Require 64-bit x86 for resctrl support
> 
>     All known hardware that supports Intel RDT or AMD QoS is 64-bit.
> 
>     On x86 the maximum supported memory size on a 32-bit kernel is 64GB (when
>     PAE is enabled). Linux imposes additional constraints since each 4KByte
>     page requires a 64-byte "struct page" which must reside in the always-mapped
>     3-4GByte range of the 32-bit kernel virtual address space.  This means the
>     maximum supported memory size is slightly above 32GB, but at that memory
>     size there are considerable inefficiencies as over half of kernel virtual
>     memory is dedicated to the page structures.
> 
>     The earliest server processor to support resctrl was Intel Haswell, released in
>     2014. It supported four memory channels, and production systems were advised
>     to populate all channels to provide sufficient memory bandwidth to keep the
>     cores busy. Best value DIMMs in 2014 were 16GB. So even a single socket Haswell
>     system would have more memory than could be addressed by a 32-bit kernel.

I assume above aims to justify the "no realistic use case for users to build a 32-bit
kernel on a server and enable resctrl"?

To be specific, my comment about RDT not being a "server" feature was made with Apollo
Lake [1] in mind. That system showed up in scenarios where servers do not typically venture
and per [1] its max memory size (8GB) is significantly less than what servers can support.

Apart from that it was the "no realistic use case" that tripped me. At best I think we
can say that we are not aware of 32-bit use cases?
>     Drop 32-bit support by changing the X86_CPU_RESCTRL dependency from X86
>     to X86_64.
> 
>     Since X86_CPU_RESCTRL now implies X86_64, remove the redundant X86_64
>     dependency from X86_CPU_RESCTRL_INTEL_AET.
> 
Reinette

[1] https://www.intel.com/content/www/us/en/products/sku/95594/intel-celeron-processor-j3455-2m-cache-up-to-2-30-ghz/specifications.html