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[84.255.245.182]) by smtp.gmail.com with ESMTPSA id ps11-20020a170906bf4b00b00a2f1888ddecsm7124755ejb.166.2024.01.24.02.39.02 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 24 Jan 2024 02:39:03 -0800 (PST) From: Uros Bizjak To: x86@kernel.org, linux-kernel@vger.kernel.org Cc: Uros Bizjak , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "H. Peter Anvin" Subject: [PATCH] x86/asm: Use 32bit xor to clear registers Date: Wed, 24 Jan 2024 11:38:59 +0100 Message-Id: <20240124103859.611372-1-ubizjak@gmail.com> X-Mailer: git-send-email 2.31.1 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" 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 Cc: Thomas Gleixner Cc: Ingo Molnar Cc: Borislav Petkov Cc: Dave Hansen Cc: "H. Peter Anvin" --- 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_G= LOBAL) ANNOTATE_NOENDBR =20 /* Clear %R15 which holds the boot_params pointer on the boot CPU */ - xorq %r15, %r15 + xorl %r15d, %r15d =20 /* * 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_G= LOBAL) #ifdef CONFIG_AMD_MEM_ENCRYPT movq sme_me_mask, %rax #else - xorq %rax, %rax + xorl %eax, %eax #endif =20 /* 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_G= LOBAL) =20 .Llookup_AP: /* EAX contains the APIC ID of the current CPU */ - xorq %rcx, %rcx + xorl %ecx, %ecx leaq cpuid_to_apicid(%rip), %rbx =20 .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 --=20 2.31.1