[tip: x86/cpu] x86/mm: Remove CONFIG_HIGHMEM64G support

tip-bot2 for Arnd Bergmann
2025-02-27T10:42:15Z

The following commit has been merged into the x86/cpu branch of tip:

Commit-ID: bbeb69ce301323e84f1677484eb8e4cd8fb1f9f8
Gitweb: https://git.kernel.org/tip/bbeb69ce301323e84f1677484eb8e4cd8fb1f9f8
Author: Arnd Bergmann <arnd@arndb.de>
AuthorDate: Wed, 26 Feb 2025 22:37:09 +01:00
Committer: Ingo Molnar <mingo@kernel.org>
CommitterDate: Thu, 27 Feb 2025 11:21:53 +01:00

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
---
Documentation/admin-guide/kdump/kdump.rst | 4 +--
Documentation/arch/x86/usb-legacy-support.rst | 11 +----
arch/x86/Kconfig | 46 ++----------------
arch/x86/configs/xen.config | 2 +-
arch/x86/include/asm/page_32_types.h | 4 +-
arch/x86/mm/init_32.c | 9 +----
6 files changed, 11 insertions(+), 65 deletions(-)

diff --git a/Documentation/admin-guide/kdump/kdump.rst b/Documentation/admin-guide/kdump/kdump.rst
index 5376890..1f7f14c 100644
--- a/Documentation/admin-guide/kdump/kdump.rst
+++ b/Documentation/admin-guide/kdump/kdump.rst
@@ -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
diff --git a/Documentation/arch/x86/usb-legacy-support.rst b/Documentation/arch/x86/usb-legacy-support.rst
index e01c08b..b17bf12 100644
--- a/Documentation/arch/x86/usb-legacy-support.rst
+++ b/Documentation/arch/x86/usb-legacy-support.rst
@@ -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
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 887b77b..737a0c6 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -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"
diff --git a/arch/x86/configs/xen.config b/arch/x86/configs/xen.config
index 5812962..d5d091e 100644
--- a/arch/x86/configs/xen.config
+++ b/arch/x86/configs/xen.config
@@ -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
diff --git a/arch/x86/include/asm/page_32_types.h b/arch/x86/include/asm/page_32_types.h
index faf9cc1..25c3265 100644
--- a/arch/x86/include/asm/page_32_types.h
+++ b/arch/x86/include/asm/page_32_types.h
@@ -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
diff --git a/arch/x86/mm/init_32.c b/arch/x86/mm/init_32.c
index ac41b1e..f288aad 100644
--- a/arch/x86/mm/init_32.c
+++ b/arch/x86/mm/init_32.c
@@ -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 */
}