linux-kernel - [PATCH 2/2] x86: rework CONFIG_X86

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Date:   Mon,  4 Dec 2023 09:47:02 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org
Cc:     Arnd Bergmann <arnd@...db.de>, "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: [PATCH 2/2] x86: rework CONFIG_X86_PAE dependency

From: Arnd Bergmann <arnd@...db.de>

While looking at a Xen Kconfig dependency issue, I tried to understand the
exact dependencies for CONFIG_X86_PAE, which is selected by CONFIG_HIGHMEM64G
but can also be enabled manually.

Apparently the dependencies for CONFIG_HIGHMEM64G are strictly about CPUs
that do support PAE, but the actual feature can be incorrectly enabled on
older CPUs as well. The CONFIG_X86_CMPXCHG64 dependencies on the other hand
include X86_PAE because cmpxchg8b is requried for PAE to work.

Rework this for readability and correctness, using a positive list of CPUs
that support PAE in a new X86_HAVE_PAE symbol that can serve as a dependency
for both X86_PAE and HIGHMEM64G as well as simplify the X86_CMPXCHG64
dependency list.

Signed-off-by: Arnd Bergmann <arnd@...db.de>
--
This did not solve the original problem of the Xen dependencies on
64 byte cache lines, as there are a number of CPUs that support PAE
but have 32 byte L1 cache lines.
---
 arch/x86/Kconfig     | 4 ++--
 arch/x86/Kconfig.cpu | 6 +++++-
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 17a6b304fc0c..c53ebc86ce4f 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1412,7 +1412,7 @@ config HIGHMEM4G
 
 config HIGHMEM64G
 	bool "64GB"
-	depends on !M486SX && !M486 && !M586 && !M586TSC && !M586MMX && !MGEODE_LX && !MGEODEGX1 && !MCYRIXIII && !MELAN && !MWINCHIPC6 && !MWINCHIP3D && !MK6
+	depends on X86_HAVE_PAE
 	select X86_PAE
 	help
 	  Select this if you have a 32-bit processor and more than 4
@@ -1469,7 +1469,7 @@ config HIGHMEM
 
 config X86_PAE
 	bool "PAE (Physical Address Extension) Support"
-	depends on X86_32 && !HIGHMEM4G
+	depends on X86_32 && X86_HAVE_PAE
 	select PHYS_ADDR_T_64BIT
 	select SWIOTLB
 	help
diff --git a/arch/x86/Kconfig.cpu b/arch/x86/Kconfig.cpu
index 00468adf180f..b9224cf2ee4d 100644
--- a/arch/x86/Kconfig.cpu
+++ b/arch/x86/Kconfig.cpu
@@ -362,9 +362,13 @@ config X86_TSC
 	def_bool y
 	depends on (MWINCHIP3D || MCRUSOE || MEFFICEON || MCYRIXIII || MK7 || MK6 || MPENTIUM4 || MPENTIUMM || MPENTIUMIII || MPENTIUMII || M686 || M586MMX || M586TSC || MK8 || MVIAC3_2 || MVIAC7 || MGEODEGX1 || MGEODE_LX || MCORE2 || MATOM) || X86_64
 
+config X86_HAVE_PAE
+	def_bool y
+	depends on MCRUSOE || MEFFICEON || MCYRIXIII || MPENTIUM4 || MPENTIUMM || MPENTIUMIII || MPENTIUMII || M686 || MK8 || MVIAC7 || MCORE2 || MATOM || X86_64
+
 config X86_CMPXCHG64
 	def_bool y
-	depends on X86_PAE || X86_64 || MCORE2 || MPENTIUM4 || MPENTIUMM || MPENTIUMIII || MPENTIUMII || M686 || M586TSC || M586MMX || MATOM || MGEODE_LX || MGEODEGX1 || MK6 || MK7 || MK8
+	depends on X86_HAVE_PAE || M586TSC || M586MMX || MK6 || MK7
 
 # this should be set for all -march=.. options where the compiler
 # generates cmov.
-- 
2.39.2