linux-kernel - [PATCH] x86/head_64: Use testb instead of testl in secondary_startup_64_no

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Date:   Thu,  9 Nov 2023 21:09:56 +0100
From:   Uros Bizjak <ubizjak@...il.com>
To:     x86@...nel.org, linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>
Subject: [PATCH] x86/head_64: Use testb instead of testl in secondary_startup_64_no_verify

There is no need to use testl when checking LSB with a test instruction.
Use testb, which is three bytes shorter:

   f6 05 00 00 00 00 01    testb  $0x1,0x0(%rip)

vs:

   f7 05 00 00 00 00 01    testl  $0x1,0x0(%rip)
   00 00 00

for the same effect.

No functional changes intended.

Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: "Peter Zijlstra (Intel)" <peterz@...radead.org>
Signed-off-by: Uros Bizjak <ubizjak@...il.com>
---
 arch/x86/kernel/head_64.S | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 086a2c3aaaa0..1f79d809305d 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -182,7 +182,7 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 	/* Enable PAE mode, PSE, PGE and LA57 */
 	orl	$(X86_CR4_PAE | X86_CR4_PSE | X86_CR4_PGE), %ecx
 #ifdef CONFIG_X86_5LEVEL
-	testl	$1, __pgtable_l5_enabled(%rip)
+	testb	$1, __pgtable_l5_enabled(%rip)
 	jz	1f
 	orl	$X86_CR4_LA57, %ecx
 1:
-- 
2.41.0