linux-kernel - Re: [PATCH] x86/boot: Remove x86

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Date:   Tue, 29 Nov 2022 09:31:17 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Uros Bizjak <ubizjak@...il.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86/boot: Remove x86_32 PIC using ebx workaround

On Tue, Nov 29, 2022 at 08:39:23AM +0100, Uros Bizjak wrote:
> On Mon, Nov 28, 2022 at 11:20 PM Borislav Petkov <bp@...en8.de> wrote:
> >
> > On Fri, Nov 04, 2022 at 01:45:46PM +0100, Uros Bizjak wrote:
> > > Current minimum required version of GCC is version 5.1 which allows
> > > reuse of PIC hard register on x86/x86-64 targets [1]. Remove
> > > obsolete workaround that was needed for earlier GCC versions.
> > >
> > > [1] https://gcc.gnu.org/gcc-5/changes.html
> >
> > Thanks for the doc pointer.
> >
> > Lemme see if I understand this commit message correctly:
> >
> > SysV i386 ABI says that %ebx is used as the base reg in PIC. gcc 5 and
> > newer can handle all possible cases properly where inline asm could
> > clobber the PIC reg. I.e., it is able to deal with the "=b" constraint
> > where an insn can overwrite %ebx and it'll push and pop around that
> > statement.
> 
> gcc-5 considers PIC register as a pseudo-register and reloads it

So not a "hard" register as you say above?

> x86_64 does not use PIC register for small code models. Also, it uses
> %r15 instead of %rbx for PIC register, so the removed workaround
> applies only to x86_32.

Let's see:

arch/x86/Makefile:
        # Never want PIC in a 32-bit kernel, prevent breakage with GCC built
        # with nonstandard options
        KBUILD_CFLAGS += -fno-pic

$ gcc -Wp,-MMD,arch/x86/boot/.cpuflags.o.d ... -fno-pic ... -D__KBUILD_MODNAME=kmod_cpuflags -c -o arch/x86/boot/cpuflags.o arch/x86/boot/cpuflags.c

So this workaround applies to nothing, I'd say. :)

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette