linux-kernel - [PATCH v4 3/6] x86/cpu: Allow caps to be set arbitrarily early

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Message-ID: <20250517091639.3807875-11-ardb+git@google.com>
Date: Sat, 17 May 2025 11:16:43 +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>, Brian Gerst <brgerst@...il.com>, 
	"Kirill A. Shutemov" <kirill@...temov.name>
Subject: [PATCH v4 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, by making sure that boot_cpu_data.x86_capability[] is
not [redundantly] wiped again when detecting CPU features.

Note that forcing a capability requires setting it in cpu_caps_set[]
too, which has been moved out of BSS in a preceding patch.

Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
 arch/x86/kernel/cpu/common.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 579d5b84e183..7392a75d85c3 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1708,9 +1708,6 @@ 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 (!cpuid_feature())
 		identify_cpu_without_cpuid(c);

@@ -1922,7 +1919,6 @@ 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));
 #ifdef CONFIG_X86_VMX_FEATURE_NAMES
 	memset(&c->vmx_capability, 0, sizeof(c->vmx_capability));
 #endif
@@ -2084,6 +2080,7 @@ void identify_secondary_cpu(unsigned int cpu)
 		*c = boot_cpu_data;
 	c->cpu_index = cpu;

+	memset(&c->x86_capability, 0, sizeof(c->x86_capability));
 	identify_cpu(c);
 #ifdef CONFIG_X86_32
 	enable_sep_cpu();
-- 
2.49.0.1101.gccaa498523-goog