Re: [tip: x86/boot] x86/boot: Use 32-bit XOR to clear registers

[Ard Biesheuvel]

On Fri, 1 Mar 2024 at 13:39, tip-bot2 for Uros Bizjak
<tip-bot2@xxxxxxxxxxxxx> wrote:
>
> The following commit has been merged into the x86/boot branch of tip:
>
> Commit-ID:     721f791ce1cddfa5f2bf524ac14741bfa0f72697
> Gitweb:        https://git.kernel.org/tip/721f791ce1cddfa5f2bf524ac14741bfa0f72697
> Author:        Uros Bizjak <ubizjak@xxxxxxxxx>
> AuthorDate:    Wed, 24 Jan 2024 11:38:59 +01:00
> Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
> CommitterDate: Fri, 01 Mar 2024 12:47:37 +01:00
>
> 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.
>

.. and so this change is pointless churn when not using legacy
registers, right?

> Slightly smaller code generated, no change in functionality.
>
> Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Brian Gerst <brgerst@xxxxxxxxx>
> Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
> Cc: H. Peter Anvin <hpa@xxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> Cc: Ard Biesheuvel <ardb@xxxxxxxxxx>
> Link: https://lore.kernel.org/r/20240124103859.611372-1-ubizjak@xxxxxxxxx
> ---
>  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 d295bf6..86136a7 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
>

   0: 4d 31 ff              xor    %r15,%r15
   3: 45 31 ff              xor    %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
>

This conflicts with my RIP-relative boot cleanup series.

>         /* 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 3355e27..1ab65f6 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