linux-kernel - [PATCH v3] x86/boot: Avoid writing to cr4 twice in startup

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Message-ID: <20250715181709.1040-1-khaliidcaliy@gmail.com>
Date: Tue, 15 Jul 2025 18:16:10 +0000
From: Khalid Ali <khaliidcaliy@...il.com>
To: tglx@...utronix.de,
	mingo@...hat.com,
	bp@...en8.de,
	dave.hansen@...ux.intel.com
Cc: x86@...nel.org,
	hpa@...or.com,
	ardb@...nel.org,
	ubizjak@...il.com,
	brgerst@...il.com,
	linux-kernel@...r.kernel.org,
	Khalid Ali <khaliidcaliy@...il.com>
Subject: [PATCH v3] x86/boot: Avoid writing to cr4 twice in startup_64()

From: Khalid Ali <khaliidcaliy@...il.com>

When Initializing cr4 bit PSE and PGE, cr4 is written twice for	
each bit. This is redundancy.

Instead, set both bits first and write CR4 once, avoiding redundant
writes. This makes consistent with cr0 writes, which is set bits and
write once.

Signed-off-by: Khalid Ali <khaliidcaliy@...il.com>
---
 arch/x86/kernel/head_64.S | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 4390a28f7dad..dfb5390e5c9a 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -222,12 +222,9 @@ SYM_INNER_LABEL(common_startup_64, SYM_L_LOCAL)
 
 	/* Even if ignored in long mode, set PSE uniformly on all logical CPUs. */
 	btsl	$X86_CR4_PSE_BIT, %ecx
-	movq	%rcx, %cr4
-
-	/*
-	 * Set CR4.PGE to re-enable global translations.
-	 */
+	/* Set CR4.PGE to re-enable global translations. */
 	btsl	$X86_CR4_PGE_BIT, %ecx
+	
 	movq	%rcx, %cr4
 
 #ifdef CONFIG_SMP
-- 
2.49.0