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

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Message-ID: <CAMzpN2joPcvg887tJLF_4SU4aJt+wTGy2M_xaExrozLS-mvXsw@mail.gmail.com>
Date: Wed, 4 Dec 2024 08:29:17 -0500
From: Brian Gerst <brgerst@...il.com>
To: Arnd Bergmann <arnd@...nel.org>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, 
	Arnd Bergmann <arnd@...db.de>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, 
	"H. Peter Anvin" <hpa@...or.com>, Linus Torvalds <torvalds@...ux-foundation.org>, 
	Andy Shevchenko <andy@...nel.org>, Matthew Wilcox <willy@...radead.org>, 
	Sean Christopherson <seanjc@...gle.com>, Davide Ciminaghi <ciminaghi@...dd.com>, 
	Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org
Subject: Re: [PATCH 05/11] x86: remove HIGHMEM64G support

On Wed, Dec 4, 2024 at 5:34 AM Arnd Bergmann <arnd@...nel.org> wrote:
>
> From: Arnd Bergmann <arnd@...db.de>
>
> 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