linux-kernel - Re: [PATCH] x86/acpi: Use %rip-relative addressing in wakeup

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Date:   Mon, 6 Nov 2023 15:14:27 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Uros Bizjak <ubizjak@...il.com>
Cc:     x86@...nel.org, linux-pm@...r.kernel.org,
        linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86/acpi: Use %rip-relative addressing in wakeup_64.S

On Fri, Nov 3, 2023 at 11:49 AM Uros Bizjak <ubizjak@...il.com> wrote:
>
> Instruction with %rip-relative address operand is one byte shorter than
> its absolute address counterpart and is also compatible with position
> independent executable (-fpie) build.
>
> No functional changes intended.

I'm wondering what's the exact motivation for making this change.

Any urgent need for it doesn't seem to be there.

> Cc: "Rafael J. Wysocki" <rafael@...nel.org>
> Cc: Len Brown <len.brown@...el.com>
> Cc: Pavel Machek <pavel@....cz>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Signed-off-by: Uros Bizjak <ubizjak@...il.com>
> ---
>  arch/x86/kernel/acpi/wakeup_64.S | 24 ++++++++++++------------
>  1 file changed, 12 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/kernel/acpi/wakeup_64.S b/arch/x86/kernel/acpi/wakeup_64.S
> index d5d8a352eafa..94ff83f3d3fe 100644
> --- a/arch/x86/kernel/acpi/wakeup_64.S
> +++ b/arch/x86/kernel/acpi/wakeup_64.S
> @@ -17,7 +17,7 @@
>          * Hooray, we are in Long 64-bit mode (but still running in low memory)
>          */
>  SYM_FUNC_START(wakeup_long64)
> -       movq    saved_magic, %rax
> +       movq    saved_magic(%rip), %rax
>         movq    $0x123456789abcdef0, %rdx
>         cmpq    %rdx, %rax
>         je      2f
> @@ -33,14 +33,14 @@ SYM_FUNC_START(wakeup_long64)
>         movw    %ax, %es
>         movw    %ax, %fs
>         movw    %ax, %gs
> -       movq    saved_rsp, %rsp
> +       movq    saved_rsp(%rip), %rsp
>
> -       movq    saved_rbx, %rbx
> -       movq    saved_rdi, %rdi
> -       movq    saved_rsi, %rsi
> -       movq    saved_rbp, %rbp
> +       movq    saved_rbx(%rip), %rbx
> +       movq    saved_rdi(%rip), %rdi
> +       movq    saved_rsi(%rip), %rsi
> +       movq    saved_rbp(%rip), %rbp
>
> -       movq    saved_rip, %rax
> +       movq    saved_rip(%rip), %rax
>         ANNOTATE_RETPOLINE_SAFE
>         jmp     *%rax
>  SYM_FUNC_END(wakeup_long64)
> @@ -72,11 +72,11 @@ SYM_FUNC_START(do_suspend_lowlevel)
>
>         movq    $.Lresume_point, saved_rip(%rip)
>
> -       movq    %rsp, saved_rsp
> -       movq    %rbp, saved_rbp
> -       movq    %rbx, saved_rbx
> -       movq    %rdi, saved_rdi
> -       movq    %rsi, saved_rsi
> +       movq    %rsp, saved_rsp(%rip)
> +       movq    %rbp, saved_rbp(%rip)
> +       movq    %rbx, saved_rbx(%rip)
> +       movq    %rdi, saved_rdi(%rip)
> +       movq    %rsi, saved_rsi(%rip)
>
>         addq    $8, %rsp
>         movl    $3, %edi
> --