From: Mateusz Jończyk <mat.jonczyk@o2.pl>
As many current platforms (most modern Intel CPUs and QEMU) have x2APIC
present, enable CONFIG_X86_X2APIC by default as it gives performance
and functionality benefits. Additionally, if the BIOS has already
switched APIC to x2APIC mode, but CONFIG_X86_X2APIC is disabled, the
kernel will panic in arch/x86/kernel/apic/apic.c .
Also improve the help text, which was confusing and really did not
describe what the feature is about.
Help text references and discussion:
Both Intel [1] and AMD [3] spell the name as "x2APIC", not "x2apic".
"It allows faster access to the local APIC"
[2], chapter 2.1, page 15:
"More efficient MSR interface to access APIC registers."
"x2APIC was introduced in Intel CPUs around 2008":
I was unable to find specific information which Intel CPUs
support x2APIC. Wikipedia claims it was "introduced with the
Nehalem microarchitecture in November 2008", but I was not able
to confirm this independently. At least some Nehalem CPUs do not
support x2APIC [1].
The documentation [2] is dated June 2008. Linux kernel also
introduced x2APIC support in 2008, so the year seems to be
right.
"and in AMD EPYC CPUs in 2019":
[3], page 15:
"AMD introduced an x2APIC in our EPYC 7002 Series processors for
the first time."
"It is also frequently emulated in virtual machines, even when the host
CPU does not support it."
[1]
"If this configuration option is disabled, the kernel will not boot on
some platforms that have x2APIC enabled."
According to some BIOS documentation [4], the x2APIC may be
"disabled", "enabled", or "force enabled" on this system.
I think that "enabled" means "made available to the operating
system, but not already turned on" and "force enabled" means
"already switched to x2APIC mode when the OS boots". Only in the
latter mode a kernel without CONFIG_X86_X2APIC will panic in
validate_x2apic() in arch/x86/kernel/apic/apic.c .
QEMU 4.2.1 and my Intel HP laptop (bought in 2019) use the
"enabled" mode and the kernel does not panic.
[1] "Re: [Qemu-devel] [Question] why x2apic's set by default without host sup"
https://lists.gnu.org/archive/html/qemu-devel/2013-07/msg03527.html
[2] Intel® 64 Architecture x2APIC Specification,
( https://www.naic.edu/~phil/software/intel/318148.pdf )
[3] Workload Tuning Guide for AMD EPYC ™ 7002 Series Processor Based
Servers Application Note,
https://developer.amd.com/wp-content/resources/56745_0.80.pdf
[4] UEFI System Utilities and Shell Command Mobile Help for HPE ProLiant
Gen10, ProLiant Gen10 Plus Servers and HPE Synergy:
Enabling or disabling Processor x2APIC Support
https://techlibrary.hpe.com/docs/iss/proliant-gen10-uefi/s_enable_disable_x2APIC_support.html
Signed-off-by: Mateusz Jończyk <mat.jonczyk@o2.pl>
---
arch/x86/Kconfig | 25 ++++++++++++++++---------
1 file changed, 16 insertions(+), 9 deletions(-)
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 5924c753097b0030fddeba246c72ff8b86e6c8ae..10f48afd67f6fd7d250f2effddcc5bc927b057af 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -462,20 +462,27 @@ config SMP
If you don't know what to do here, say N.
config X86_X2APIC
- bool "Support x2apic"
+ bool "x2APIC interrupt controller architecture support"
depends on X86_LOCAL_APIC && X86_64 && (IRQ_REMAP || HYPERVISOR_GUEST)
+ default y
help
- This enables x2apic support on CPUs that have this feature.
+ x2APIC is an interrupt controller architecture, a component of which
+ (the local APIC) is present in the CPU. It allows faster access to
+ the local APIC and supports a larger number of CPUs in the system
+ than the predecessors.
- This allows 32-bit apic IDs (so it can support very large systems),
- and accesses the local apic via MSRs not via mmio.
+ x2APIC was introduced in Intel CPUs around 2008 and in AMD EPYC CPUs
+ in 2019, but it can be disabled by the BIOS. It is also frequently
+ emulated in virtual machines, even when the host CPU does not support
+ it. Support in the CPU can be checked by executing
+ cat /proc/cpuinfo | grep x2apic
- Some Intel systems circa 2022 and later are locked into x2APIC mode
- and can not fall back to the legacy APIC modes if SGX or TDX are
- enabled in the BIOS. They will boot with very reduced functionality
- without enabling this option.
+ If this configuration option is disabled, the kernel will not boot on
+ some platforms that have x2APIC enabled.
- If you don't know what to do here, say N.
+ Say N if you know that your platform does not have x2APIC.
+
+ Otherwise, say Y.
config X86_POSTED_MSI
bool "Enable MSI and MSI-x delivery by posted interrupts"
--
2.49.0
The following commit has been merged into the x86/urgent branch of tip:
Commit-ID: 9232c49ff31c40fa5cf72acf74c4aa251ed4811c
Gitweb: https://git.kernel.org/tip/9232c49ff31c40fa5cf72acf74c4aa251ed4811c
Author: Mateusz Jończyk <mat.jonczyk@o2.pl>
AuthorDate: Fri, 21 Mar 2025 21:48:43 +01:00
Committer: Ingo Molnar <mingo@kernel.org>
CommitterDate: Sat, 22 Mar 2025 08:02:16 +01:00
x86/Kconfig: Enable X86_X2APIC by default and improve help text
As many current platforms (most modern Intel CPUs and QEMU) have x2APIC
present, enable CONFIG_X86_X2APIC by default as it gives performance
and functionality benefits. Additionally, if the BIOS has already
switched APIC to x2APIC mode, but CONFIG_X86_X2APIC is disabled, the
kernel will panic in arch/x86/kernel/apic/apic.c .
Also improve the help text, which was confusing and really did not
describe what the feature is about.
Help text references and discussion:
Both Intel [1] and AMD [3] spell the name as "x2APIC", not "x2apic".
"It allows faster access to the local APIC"
[2], chapter 2.1, page 15:
"More efficient MSR interface to access APIC registers."
"x2APIC was introduced in Intel CPUs around 2008":
I was unable to find specific information which Intel CPUs
support x2APIC. Wikipedia claims it was "introduced with the
Nehalem microarchitecture in November 2008", but I was not able
to confirm this independently. At least some Nehalem CPUs do not
support x2APIC [1].
The documentation [2] is dated June 2008. Linux kernel also
introduced x2APIC support in 2008, so the year seems to be
right.
"and in AMD EPYC CPUs in 2019":
[3], page 15:
"AMD introduced an x2APIC in our EPYC 7002 Series processors for
the first time."
"It is also frequently emulated in virtual machines, even when the host
CPU does not support it."
[1]
"If this configuration option is disabled, the kernel will not boot on
some platforms that have x2APIC enabled."
According to some BIOS documentation [4], the x2APIC may be
"disabled", "enabled", or "force enabled" on this system.
I think that "enabled" means "made available to the operating
system, but not already turned on" and "force enabled" means
"already switched to x2APIC mode when the OS boots". Only in the
latter mode a kernel without CONFIG_X86_X2APIC will panic in
validate_x2apic() in arch/x86/kernel/apic/apic.c .
QEMU 4.2.1 and my Intel HP laptop (bought in 2019) use the
"enabled" mode and the kernel does not panic.
[1] "Re: [Qemu-devel] [Question] why x2apic's set by default without host sup"
https://lists.gnu.org/archive/html/qemu-devel/2013-07/msg03527.html
[2] Intel® 64 Architecture x2APIC Specification,
( https://www.naic.edu/~phil/software/intel/318148.pdf )
[3] Workload Tuning Guide for AMD EPYC ™ 7002 Series Processor Based
Servers Application Note,
https://developer.amd.com/wp-content/resources/56745_0.80.pdf
[4] UEFI System Utilities and Shell Command Mobile Help for HPE ProLiant
Gen10, ProLiant Gen10 Plus Servers and HPE Synergy:
Enabling or disabling Processor x2APIC Support
https://techlibrary.hpe.com/docs/iss/proliant-gen10-uefi/s_enable_disable_x2APIC_support.html
Signed-off-by: Mateusz Jończyk <mat.jonczyk@o2.pl>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20250321-x86_x2apic-v3-1-b0cbaa6fa338@ixit.cz
---
arch/x86/Kconfig | 25 ++++++++++++++++---------
1 file changed, 16 insertions(+), 9 deletions(-)
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 98bd493..08bc939 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -458,20 +458,27 @@ config SMP
If you don't know what to do here, say N.
config X86_X2APIC
- bool "Support x2apic"
+ bool "x2APIC interrupt controller architecture support"
depends on X86_LOCAL_APIC && X86_64 && (IRQ_REMAP || HYPERVISOR_GUEST)
+ default y
help
- This enables x2apic support on CPUs that have this feature.
+ x2APIC is an interrupt controller architecture, a component of which
+ (the local APIC) is present in the CPU. It allows faster access to
+ the local APIC and supports a larger number of CPUs in the system
+ than the predecessors.
- This allows 32-bit apic IDs (so it can support very large systems),
- and accesses the local apic via MSRs not via mmio.
+ x2APIC was introduced in Intel CPUs around 2008 and in AMD EPYC CPUs
+ in 2019, but it can be disabled by the BIOS. It is also frequently
+ emulated in virtual machines, even when the host CPU does not support
+ it. Support in the CPU can be checked by executing
+ cat /proc/cpuinfo | grep x2apic
- Some Intel systems circa 2022 and later are locked into x2APIC mode
- and can not fall back to the legacy APIC modes if SGX or TDX are
- enabled in the BIOS. They will boot with very reduced functionality
- without enabling this option.
+ If this configuration option is disabled, the kernel will not boot on
+ some platforms that have x2APIC enabled.
- If you don't know what to do here, say N.
+ Say N if you know that your platform does not have x2APIC.
+
+ Otherwise, say Y.
config X86_POSTED_MSI
bool "Enable MSI and MSI-x delivery by posted interrupts"Currently, it is not true that the kernel will panic with CONFIG_X86_X2APIC=n
on systems that require it; it will try to disable the APIC and run without
it to at least give the user a clear warning message. See the second
variant of check_x2apic() in arch/x86/kernel/apic/apic.c .
Also massage some other parts of the help text.
Signed-off-by: Mateusz Jończyk <mat.jonczyk@o2.pl>
Fixes: 9232c49ff31c ("x86/Kconfig: Enable X86_X2APIC by default and improve help text")
Cc: David Heidelberg <david@ixit.cz>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
---
This patch fixes a patch currently in tip.git master and depends on it.
I have not verified recently that the kernel will work at all without
CONFIG_X86_X2APIC on systems which force enable it - but it was working
in 2022.
---
arch/x86/Kconfig | 13 +++++++------
1 file changed, 7 insertions(+), 6 deletions(-)
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index e72cb7779038..ef48584c8889 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -471,14 +471,15 @@ config X86_X2APIC
in 2019, but it can be disabled by the BIOS. It is also frequently
emulated in virtual machines, even when the host CPU does not support
it. Support in the CPU can be checked by executing
- cat /proc/cpuinfo | grep x2apic
+ grep x2apic /proc/cpuinfo
- If this configuration option is disabled, the kernel will not boot on
- some platforms that have x2APIC enabled.
+ If this configuration option is disabled, the kernel will boot with
+ very reduced functionality and performance on some platforms that
+ have x2APIC enabled. On the other hand, on hardware that does not
+ support x2APIC, a kernel with this option enabled will just fallback
+ to older APIC implementations.
- Say N if you know that your platform does not have x2APIC.
-
- Otherwise, say Y.
+ If in doubt, say Y.
config X86_POSTED_MSI
bool "Enable MSI and MSI-x delivery by posted interrupts"
--
2.25.1
The following commit has been merged into the x86/urgent branch of tip:
Commit-ID: 99bb1bd810eaf37e15ef757a30a815e774a2445b
Gitweb: https://git.kernel.org/tip/99bb1bd810eaf37e15ef757a30a815e774a2445b
Author: Mateusz Jończyk <mat.jonczyk@o2.pl>
AuthorDate: Sat, 22 Mar 2025 16:45:41 +01:00
Committer: Ingo Molnar <mingo@kernel.org>
CommitterDate: Tue, 25 Mar 2025 08:17:49 +01:00
x86/Kconfig: Correct X86_X2APIC help text
Currently, it is not true that the kernel will panic with CONFIG_X86_X2APIC=n
on systems that require it; it will try to disable the APIC and run without
it to at least give the user a clear warning message. See the second
variant of check_x2apic() in arch/x86/kernel/apic/apic.c .
Also massage some other parts of the help text.
Fixes: 9232c49ff31c ("x86/Kconfig: Enable X86_X2APIC by default and improve help text")
Signed-off-by: Mateusz Jończyk <mat.jonczyk@o2.pl>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250322154541.40325-1-mat.jonczyk@o2.pl
---
arch/x86/Kconfig | 13 +++++++------
1 file changed, 7 insertions(+), 6 deletions(-)
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index e72cb77..ef48584 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -471,14 +471,15 @@ config X86_X2APIC
in 2019, but it can be disabled by the BIOS. It is also frequently
emulated in virtual machines, even when the host CPU does not support
it. Support in the CPU can be checked by executing
- cat /proc/cpuinfo | grep x2apic
+ grep x2apic /proc/cpuinfo
- If this configuration option is disabled, the kernel will not boot on
- some platforms that have x2APIC enabled.
+ If this configuration option is disabled, the kernel will boot with
+ very reduced functionality and performance on some platforms that
+ have x2APIC enabled. On the other hand, on hardware that does not
+ support x2APIC, a kernel with this option enabled will just fallback
+ to older APIC implementations.
- Say N if you know that your platform does not have x2APIC.
-
- Otherwise, say Y.
+ If in doubt, say Y.
config X86_POSTED_MSI
bool "Enable MSI and MSI-x delivery by posted interrupts"© 2016 - 2026 Red Hat, Inc.