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 <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20240124103859.611372-1-ubizjak@gmail.com

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index d295bf68bf944f231fa2876e62015f898b534324..86136a78778e85f730f8573a0d13b718f3772a8b 100644 (file)
--- 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 3355e27c69ebf8b51be11d479f00795fd3935a19..1ab65f6c6ae7a159221b438a7acf5d0df4740618 100644 (file)
--- 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