Re: [PATCH 05/11] x86: remove HIGHMEM64G support

[Brian Gerst]

On Wed, Dec 4, 2024 at 5:34 AM Arnd Bergmann <arnd@xxxxxxxxxx> wrote:
>
> From: Arnd Bergmann <arnd@xxxxxxxx>
>
> The HIGHMEM64G support was added in linux-2.3.25 to support (then)
> high-end Pentium Pro and Pentium III Xeon servers with more than 4GB of
> addressing, NUMA and PCI-X slots started appearing.
>
> I have found no evidence of this ever being used in regular dual-socket
> servers or consumer devices, all the users seem obsolete these days,
> even by i386 standards:
>
>  - Support for NUMA servers (NUMA-Q, IBM x440, unisys) was already
>    removed ten years ago.
>
>  - 4+ socket non-NUMA servers based on Intel 450GX/450NX, HP F8 and
>    ServerWorks ServerSet/GrandChampion could theoretically still work
>    with 8GB, but these were exceptionally rare even 20 years ago and
>    would have usually been equipped with than the maximum amount of
>    RAM.
>
>  - Some SKUs of the Celeron D from 2004 had 64-bit mode fused off but
>    could still work in a Socket 775 mainboard designed for the later
>    Core 2 Duo and 8GB. Apparently most BIOSes at the time only allowed
>    64-bit CPUs.
>
>  - In the early days of x86-64 hardware, there was sometimes the need
>    to run a 32-bit kernel to work around bugs in the hardware drivers,
>    or in the syscall emulation for 32-bit userspace. This likely still
>    works but there should never be a need for this any more.
>
> Removing this also drops the need for PHYS_ADDR_T_64BIT and SWIOTLB.
> PAE mode is still required to get access to the 'NX' bit on Atom
> 'Pentium M' and 'Core Duo' CPUs.

8GB of memory is still useful for 32-bit guest VMs.


Brian Gerst