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

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Date: Fri, 1 Mar 2024 13:50:47 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org, 
	Ingo Molnar <mingo@...nel.org>, Andy Lutomirski <luto@...nel.org>, Brian Gerst <brgerst@...il.com>, 
	Denys Vlasenko <dvlasenk@...hat.com>, "H. Peter Anvin" <hpa@...or.com>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org
Subject: Re: [tip: x86/boot] x86/boot: Use 32-bit XOR to clear registers

On Fri, Mar 1, 2024 at 1:45 PM Ard Biesheuvel <ardb@...nel.org> wrote:
>
> On Fri, 1 Mar 2024 at 13:39, tip-bot2 for Uros Bizjak
> <tip-bot2@...utronix.de> 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@...il.com>
> > AuthorDate:    Wed, 24 Jan 2024 11:38:59 +01:00
> > Committer:     Ingo Molnar <mingo@...nel.org>
> > 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?

Although there is no code size change with REX registers, it would
look weird to use XORQ with REX registers and XORL with legacy regs.
Please see arch/x86/kvm/{vmx,svm}/vmenter.S where this approach is
also used.

Uros.

> > Slightly smaller code generated, no change in functionality.
> >
> > Signed-off-by: Uros Bizjak <ubizjak@...il.com>
> > Signed-off-by: Ingo Molnar <mingo@...nel.org>
> > Cc: Andy Lutomirski <luto@...nel.org>
> > Cc: Brian Gerst <brgerst@...il.com>
> > Cc: Denys Vlasenko <dvlasenk@...hat.com>
> > Cc: H. Peter Anvin <hpa@...or.com>
> > Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> > Cc: Josh Poimboeuf <jpoimboe@...hat.com>
> > Cc: Ard Biesheuvel <ardb@...nel.org>
> > 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 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