Re: [PATCH] x86/boot: Remove x86_32 PIC using ebx workaround

[Uros Bizjak]

On Mon, Nov 28, 2022 at 11:20 PM Borislav Petkov <bp@xxxxxxxxx> 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
before instruction that requires the value in %ebx. This way, PIC
register is no more special and does not need any special handling.
This includes inline asm which can clobber %ebx.
>
> So far so good.
>
> Why then does this matter for x86-64 where PIC addressing is done
> rip-relative so %rbx is normal reg there?

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.

Uros.