From 721f791ce1cddfa5f2bf524ac14741bfa0f72697 Mon Sep 17 00:00:00 2001 From: Uros Bizjak Date: Wed, 24 Jan 2024 11:38:59 +0100 Subject: [PATCH] x86/boot: Use 32-bit XOR to clear registers x86_64 zero extends 32-bit operations, so for 64-bit operands, XORL r32,r32 is functionally equal to XORQ r64,r64, but avoids a REX prefix byte when legacy registers are used. Slightly smaller code generated, no change in functionality. Signed-off-by: Uros Bizjak Signed-off-by: Ingo Molnar Cc: Andy Lutomirski Cc: Brian Gerst Cc: Denys Vlasenko Cc: H. Peter Anvin Cc: Linus Torvalds Cc: Josh Poimboeuf Cc: Ard Biesheuvel Link: https://lore.kernel.org/r/20240124103859.611372-1-ubizjak@gmail.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 d295bf68bf944..86136a78778e8 100644 --- a/arch/x86/kernel/head_64.S +++ b/arch/x86/kernel/head_64.S @@ -169,7 +169,7 @@ 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 @@ -178,7 +178,7 @@ 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 */ @@ -295,7 +295,7 @@ 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 3355e27c69ebf..1ab65f6c6ae7a 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 -- 2.30.2