From: Uros Bizjak <ubizjak@gmail.com>
To: x86@kernel.org, linux-kernel@vger.kernel.org
Cc: Uros Bizjak <ubizjak@gmail.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>,
"H. Peter Anvin" <hpa@zytor.com>
Subject: [PATCH] x86/asm: Use 32bit xor to clear registers
Date: Wed, 24 Jan 2024 11:38:59 +0100 [thread overview]
Message-ID: <20240124103859.611372-1-ubizjak@gmail.com> (raw)
x86_64 zero extends 32bit operations, so for 64bit operands,
XORL r32,r32 is functionally equal to XORQ r64,r64, but avoids
a REX prefix byte when legacy registers are used.
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
---
arch/x86/kernel/head_64.S | 6 +++---
arch/x86/kernel/sev_verify_cbit.S | 2 +-
2 files changed, 4 insertions(+), 4 deletions(-)
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index cc3a81852e4a..ed287170c126 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -171,7 +171,7 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
ANNOTATE_NOENDBR
/* Clear %R15 which holds the boot_params pointer on the boot CPU */
- xorq %r15, %r15
+ xorl %r15d, %r15d
/*
* Retrieve the modifier (SME encryption mask if SME is active) to be
@@ -180,7 +180,7 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
#ifdef CONFIG_AMD_MEM_ENCRYPT
movq sme_me_mask, %rax
#else
- xorq %rax, %rax
+ xorl %eax, %eax
#endif
/* Form the CR3 value being sure to include the CR3 modifier */
@@ -297,7 +297,7 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
.Llookup_AP:
/* EAX contains the APIC ID of the current CPU */
- xorq %rcx, %rcx
+ xorl %ecx, %ecx
leaq cpuid_to_apicid(%rip), %rbx
.Lfind_cpunr:
diff --git a/arch/x86/kernel/sev_verify_cbit.S b/arch/x86/kernel/sev_verify_cbit.S
index 3355e27c69eb..1ab65f6c6ae7 100644
--- a/arch/x86/kernel/sev_verify_cbit.S
+++ b/arch/x86/kernel/sev_verify_cbit.S
@@ -77,7 +77,7 @@ SYM_FUNC_START(sev_verify_cbit)
* The check failed, prevent any forward progress to prevent ROP
* attacks, invalidate the stack and go into a hlt loop.
*/
- xorq %rsp, %rsp
+ xorl %esp, %esp
subq $0x1000, %rsp
2: hlt
jmp 2b
--
2.31.1
next reply other threads:[~2024-01-24 10:39 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-01-24 10:38 Uros Bizjak [this message]
2024-03-01 12:39 ` [tip: x86/boot] x86/boot: Use 32-bit XOR to clear registers tip-bot2 for Uros Bizjak
2024-03-01 12:44 ` Ard Biesheuvel
2024-03-01 12:50 ` Uros Bizjak
2024-03-01 13:10 ` Ard Biesheuvel
2024-03-01 16:02 ` Andrew Cooper
2024-03-01 16:35 ` Ard Biesheuvel
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