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 <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Brian Gerst <[e-mail]>
Cc: Denys Vlasenko <[e-mail]>
Cc: H. Peter Anvin <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Josh Poimboeuf <[e-mail]>
Cc: Ard Biesheuvel <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

arch/x86/kernel/head_64.S

···
 	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
···
 #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 */
···
 
 .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:

arch/x86/kernel/sev_verify_cbit.S

···
 	 * 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