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

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Message-ID: <CAFULd4YPVcWL6_bo2T4O=a6ed9hTOakp2m1_XBF3w-uWhyd4dQ@mail.gmail.com>
Date:   Mon, 6 Nov 2023 15:25:43 +0100
From:   Uros Bizjak <ubizjak@...il.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     x86@...nel.org, linux-pm@...r.kernel.org,
        linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.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 Mon, Nov 6, 2023 at 3:14 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> 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.

Mainly to be consistent with what the compiler emits by default when a
symbol is accessed. As said in the commit message, the %rip-relative
access is also one byte shorter, and results in a position independent
code.

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

True. It's mostly a nice-to-have change.

Thanks,
Uros.

> > 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
> > --