linux-kernel - Re: [PATCH v5 6/6] x86/asm: Retire RIP_REL

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Message-ID: <CAFULd4b3uHB+tvTXroDP-MLgpBHtGxMZgxCp3FVpN7bx=Ov+7w@mail.gmail.com>
Date: Sun, 4 May 2025 09:33:04 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org, 
	linux-efi@...r.kernel.org, x86@...nel.org, mingo@...nel.org, 
	Tom Lendacky <thomas.lendacky@....com>, Dionna Amalie Glaze <dionnaglaze@...gle.com>, 
	Kevin Loughlin <kevinloughlin@...gle.com>
Subject: Re: [PATCH v5 6/6] x86/asm: Retire RIP_REL_REF()

On Fri, Apr 18, 2025 at 5:58 PM Ard Biesheuvel <ardb@...nel.org> wrote:
>
> On Fri, 18 Apr 2025 at 17:51, Uros Bizjak <ubizjak@...il.com> wrote:
> >
> >
> >
> > On 18. 04. 25 16:13, Ard Biesheuvel wrote:
> > > From: Ard Biesheuvel <ardb@...nel.org>
> > >
> > > Now that all users have been moved into startup/ where PIC codegen is
> > > used, RIP_REL_REF() is no longer needed. Remove it.
> > >
> > > Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> > > ---
> > >   arch/x86/include/asm/asm.h | 5 -----
> > >   1 file changed, 5 deletions(-)
> > >
> > > diff --git a/arch/x86/include/asm/asm.h b/arch/x86/include/asm/asm.h
> > > index a9f07799e337..eef0771512de 100644
> > > --- a/arch/x86/include/asm/asm.h
> > > +++ b/arch/x86/include/asm/asm.h
> > > @@ -120,11 +120,6 @@ static __always_inline __pure void *rip_rel_ptr(void *p)
> > >
> > >       return p;
> > >   }
> > > -#ifndef __pic__
> > > -#define RIP_REL_REF(var)     (*(typeof(&(var)))rip_rel_ptr(&(var)))
> > > -#else
> > > -#define RIP_REL_REF(var)     (var)
> > > -#endif
> > >   #endif
> >
> > You can also remove rip_rel_ptr() with the whole "#ifndef __ASSEMBLER__"
> > part.
> >
>
> No, rip_rel_ptr() needs to be kept, unfortunately.

Indeed, I scanned the source before your patch, where rip_rel_ptr()
was used exclusively via RIP_REL_REF.

On a related note,

asm("leaq %c1(%%rip), %0" : "=r"(p) : "i"(p));

the above asm can use %a1 generic operand modifier instead of
"%c1(%%rip)" [1], similar to d689863c1a60b9 [2], which also explains
the modifier:

--q--
The "a" asm operand modifier substitutes a memory reference, with the
actual operand treated as address. For x86_64, when a symbol is
provided, the "a" modifier emits "sym(%rip)" instead of "sym",
enabling shorter %rip-relative addressing.
--/q--

[1] https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#Generic-Operand-Modifiers
[2] https://web.git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/arch/x86/include/asm?id=d689863c1a60b9936b47a34fa5c3330de374f4fc

Uros.