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x86/mm: Remove CONFIG_HIGHMEM64G support 

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.

 - The rare Xeon LV "Sossaman" came on a few motherboards with
   registered DDR2 memory support up to 16GB.

 - 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.

PAE mode is still required to get access to the 'NX' bit on Atom
'Pentium M' and 'Core Duo' CPUs.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20250226213714.4040853-6-arnd@kernel.org
This commit is contained in:
Arnd Bergmann 2025-02-26 22:37:09 +01:00 committed by Ingo Molnar
parent f388f60ca9
commit bbeb69ce30
6 changed files with 11 additions and 65 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
Documentation/admin-guide/kdump/kdump.rst
View File

@ -180,10 +180,6 @@ Dump-capture kernel config options (Arch Dependent, i386 and x86_64)
1) On i386, enable high memory support under "Processor type and
features"::
CONFIG_HIGHMEM64G=y
or::
CONFIG_HIGHMEM4G
2) With CONFIG_SMP=y, usually nr_cpus=1 need specified on the kernel

11
Documentation/arch/x86/usb-legacy-support.rst
View File

@ -20,11 +20,7 @@ It has several drawbacks, though:
features (wheel, extra buttons, touchpad mode) of the real PS/2 mouse may
not be available.
2) If CONFIG_HIGHMEM64G is enabled, the PS/2 mouse emulation can cause
system crashes, because the SMM BIOS is not expecting to be in PAE mode.
The Intel E7505 is a typical machine where this happens.
3) If AMD64 64-bit mode is enabled, again system crashes often happen,
2) If AMD64 64-bit mode is enabled, again system crashes often happen,
because the SMM BIOS isn't expecting the CPU to be in 64-bit mode. The
BIOS manufacturers only test with Windows, and Windows doesn't do 64-bit
yet.
@ -38,11 +34,6 @@ Problem 1)
compiled-in, too.
Problem 2)
can currently only be solved by either disabling HIGHMEM64G
in the kernel config or USB Legacy support in the BIOS. A BIOS update
could help, but so far no such update exists.
Problem 3)
is usually fixed by a BIOS update. Check the board
manufacturers web site. If an update is not available, disable USB
Legacy support in the BIOS. If this alone doesn't help, try also adding

46
arch/x86/Kconfig
View File

@ -1388,15 +1388,11 @@ config X86_CPUID
with major 203 and minors 0 to 31 for /dev/cpu/0/cpuid to
/dev/cpu/31/cpuid.
choice
prompt "High Memory Support"
default HIGHMEM4G
config HIGHMEM4G
bool "High Memory Support"
depends on X86_32
config NOHIGHMEM
bool "off"
help
Linux can use up to 64 Gigabytes of physical memory on x86 systems.
Linux can use up to 4 Gigabytes of physical memory on x86 systems.
However, the address space of 32-bit x86 processors is only 4
Gigabytes large. That means that, if you have a large amount of
physical memory, not all of it can be "permanently mapped" by the
@ -1412,38 +1408,9 @@ config NOHIGHMEM
possible.
If the machine has between 1 and 4 Gigabytes physical RAM, then
answer "4GB" here.
answer "Y" here.
If more than 4 Gigabytes is used then answer "64GB" here. This
selection turns Intel PAE (Physical Address Extension) mode on.
PAE implements 3-level paging on IA32 processors. PAE is fully
supported by Linux, PAE mode is implemented on all recent Intel
processors (Pentium Pro and better). NOTE: If you say "64GB" here,
then the kernel will not boot on CPUs that don't support PAE!
The actual amount of total physical memory will either be
auto detected or can be forced by using a kernel command line option
such as "mem=256M". (Try "man bootparam" or see the documentation of
your boot loader (lilo or loadlin) about how to pass options to the
kernel at boot time.)
If unsure, say "off".
config HIGHMEM4G
bool "4GB"
help
Select this if you have a 32-bit processor and between 1 and 4
gigabytes of physical RAM.
config HIGHMEM64G
bool "64GB"
depends on X86_HAVE_PAE
select X86_PAE
help
Select this if you have a 32-bit processor and more than 4
gigabytes of physical RAM.
endchoice
If unsure, say N.
choice
prompt "Memory split" if EXPERT
@ -1489,8 +1456,7 @@ config PAGE_OFFSET
depends on X86_32
config HIGHMEM
def_bool y
depends on X86_32 && (HIGHMEM64G || HIGHMEM4G)
def_bool HIGHMEM4G
config X86_PAE
bool "PAE (Physical Address Extension) Support"

2
arch/x86/configs/xen.config
View File

@ -1,6 +1,4 @@
# global x86 required specific stuff
# On 32-bit HIGHMEM4G is not allowed
CONFIG_HIGHMEM64G=y
CONFIG_64BIT=y
# These enable us to allow some of the

4
arch/x86/include/asm/page_32_types.h
View File

@ -11,8 +11,8 @@
* a virtual address space of one gigabyte, which limits the
* amount of physical memory you can use to about 950MB.
*
* If you want more physical memory than this then see the CONFIG_HIGHMEM4G
* and CONFIG_HIGHMEM64G options in the kernel configuration.
* If you want more physical memory than this then see the CONFIG_VMSPLIT_2G
* and CONFIG_HIGHMEM4G options in the kernel configuration.
*/
#define __PAGE_OFFSET_BASE _AC(CONFIG_PAGE_OFFSET, UL)
#define __PAGE_OFFSET __PAGE_OFFSET_BASE

9
arch/x86/mm/init_32.c
View File

@ -582,7 +582,7 @@ static void __init lowmem_pfn_init(void)
"only %luMB highmem pages available, ignoring highmem size of %luMB!\n"
#define MSG_HIGHMEM_TRIMMED \
"Warning: only 4GB will be used. Use a HIGHMEM64G enabled kernel!\n"
"Warning: only 4GB will be used. Support for for CONFIG_HIGHMEM64G was removed!\n"
/*
* We have more RAM than fits into lowmem - we try to put it into
* highmem, also taking the highmem=x boot parameter into account:
@ -606,18 +606,13 @@ static void __init highmem_pfn_init(void)
#ifndef CONFIG_HIGHMEM
/* Maximum memory usable is what is directly addressable */
printk(KERN_WARNING "Warning only %ldMB will be used.\n", MAXMEM>>20);
if (max_pfn > MAX_NONPAE_PFN)
printk(KERN_WARNING "Use a HIGHMEM64G enabled kernel.\n");
else
printk(KERN_WARNING "Use a HIGHMEM enabled kernel.\n");
printk(KERN_WARNING "Use a HIGHMEM enabled kernel.\n");
max_pfn = MAXMEM_PFN;
#else /* !CONFIG_HIGHMEM */
#ifndef CONFIG_HIGHMEM64G
if (max_pfn > MAX_NONPAE_PFN) {
max_pfn = MAX_NONPAE_PFN;
printk(KERN_WARNING MSG_HIGHMEM_TRIMMED);
}
#endif /* !CONFIG_HIGHMEM64G */
#endif /* !CONFIG_HIGHMEM */
}