linux-kernel - [PATCH] x86/Kconfig: correct X86

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Message-Id: <20250322154541.40325-1-mat.jonczyk@o2.pl>
Date: Sat, 22 Mar 2025 16:45:41 +0100
From: Mateusz Jończyk <mat.jonczyk@...pl>
To: x86@...nel.org,
	linux-kernel@...r.kernel.org
Cc: Mateusz Jończyk <mat.jonczyk@...pl>,
	David Heidelberg <david@...t.cz>,
	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>
Subject: [PATCH] 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.

Signed-off-by: Mateusz Jończyk <mat.jonczyk@...pl>
Fixes: 9232c49ff31c ("x86/Kconfig: Enable X86_X2APIC by default and improve help text")
Cc: David Heidelberg <david@...t.cz>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: "H. Peter Anvin" <hpa@...or.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