linux-kernel - [RFC PATCH v2 3/6] x86/cpu: Allow caps to be set arbitrarily early

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Message-ID: <20250513111157.717727-11-ardb+git@google.com>
Date: Tue, 13 May 2025 13:12:01 +0200
From: Ard Biesheuvel <ardb+git@...gle.com>
To: linux-kernel@...r.kernel.org
Cc: x86@...nel.org, Ard Biesheuvel <ardb@...nel.org>, Ingo Molnar <mingo@...nel.org>, 
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [RFC PATCH v2 3/6] x86/cpu: Allow caps to be set arbitrarily early

From: Ard Biesheuvel <ardb@...nel.org>

cpu_feature_enabled() uses a ternary alternative, where the late variant
is based on code patching and the early variant accesses the capability
field in boot_cpu_data directly.

This allows cpu_feature_enabled() to be called quite early, but it still
requires that the CPU feature detection code runs before being able to
rely on the return value of cpu_feature_enabled().

This is a problem for the implementation of pgtable_l5_enabled(), which
is based on cpu_feature_enabled(X86_FEATURE_5LEVEL_PAGING), and may be
called extremely early. Currently, there is a hacky workaround where
some source files that may execute before (but also after) CPU feature
detection have a different version of pgtable_l5_enabled(), based on the
USE_EARLY_PGTABLE_L5 preprocessor macro.

Instead, let's make it possible to set CPU feature arbitrarily early, so
that the X86_FEATURE_5LEVEL_PAGING capability can be set before even
entering C code.

This involves relying on static initialization of boot_cpu_data and the
cpu_caps_set/cpu_caps_cleared arrays, so they all need to reside in
.data. This ensures that they won't be cleared along with the rest of
BSS.

Note that forcing a capability involves setting it in both
boot_cpu_data.x86_capability[] and cpu_caps_set[].

Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
 arch/x86/kernel/cpu/common.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index bbec5c4cd8ed..aaa6d9e51ef1 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -704,8 +704,8 @@ static const char *table_lookup_model(struct cpuinfo_x86 *c)
 }

 /* Aligned to unsigned long to avoid split lock in atomic bitmap ops */
-__u32 cpu_caps_cleared[NCAPINTS + NBUGINTS] __aligned(sizeof(unsigned long));
-__u32 cpu_caps_set[NCAPINTS + NBUGINTS] __aligned(sizeof(unsigned long));
+__u32 __read_mostly cpu_caps_cleared[NCAPINTS + NBUGINTS] __aligned(sizeof(unsigned long));
+__u32 __read_mostly cpu_caps_set[NCAPINTS + NBUGINTS] __aligned(sizeof(unsigned long));

 #ifdef CONFIG_X86_32
 /* The 32-bit entry code needs to find cpu_entry_area. */
@@ -1628,9 +1628,6 @@ static void __init cpu_parse_early_param(void)
  */
 static void __init early_identify_cpu(struct cpuinfo_x86 *c)
 {
-	memset(&c->x86_capability, 0, sizeof(c->x86_capability));
-	c->extended_cpuid_level = 0;
-
 	if (!have_cpuid_p())
 		identify_cpu_without_cpuid(c);

@@ -1842,7 +1839,8 @@ static void identify_cpu(struct cpuinfo_x86 *c)
 	c->x86_virt_bits = 32;
 #endif
 	c->x86_cache_alignment = c->x86_clflush_size;
-	memset(&c->x86_capability, 0, sizeof(c->x86_capability));
+	if (c != &boot_cpu_data)
+		memset(&c->x86_capability, 0, sizeof(c->x86_capability));
 #ifdef CONFIG_X86_VMX_FEATURE_NAMES
 	memset(&c->vmx_capability, 0, sizeof(c->vmx_capability));
 #endif
-- 
2.49.0.1045.g170613ef41-goog