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

[Luck, Tony]

> 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.

    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.

-Tony