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From: Borislav Petkov <bp@alien8.de>
To: Uros Bizjak <ubizjak@gmail.com>
Cc: x86@kernel.org, linux-kernel@vger.kernel.org,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH] x86/boot: Remove x86_32 PIC using ebx workaround
Date: Mon, 28 Nov 2022 23:20:11 +0100	[thread overview]
Message-ID: <Y4U0GwlLgAuxu3WF@zn.tnic> (raw)
In-Reply-To: <20221104124546.196077-1-ubizjak@gmail.com>

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.

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?

Thx.

-- 
Regards/Gruss,
    Boris.

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

  reply	other threads:[~2022-11-28 22:20 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-11-04 12:45 Uros Bizjak
2022-11-28 22:20 ` Borislav Petkov [this message]
2022-11-29  3:41   ` Brian Gerst
2022-11-29  8:32     ` Borislav Petkov
2022-11-29 13:24       ` Brian Gerst
2022-11-29 13:26         ` Uros Bizjak
2022-11-29  7:39   ` Uros Bizjak
2022-11-29  8:31     ` Borislav Petkov
2022-11-29  8:50       ` Uros Bizjak
2022-11-29 14:46         ` Borislav Petkov
2022-11-29 14:56           ` Uros Bizjak
2022-11-29 15:27             ` Borislav Petkov
2022-11-29 19:44 ` [tip: x86/boot] x86/boot: Remove x86_32 PIC using %ebx workaround tip-bot2 for Uros Bizjak

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