linux-kernel - Re: [PATCH -tip] x86: Remove code depending on __GCC_ASM_FLAG_OUTPUTS_

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Message-ID: <20250905131909.GDaLrjTTw0PyFHeCHp@fat_crate.local>
Date: Fri, 5 Sep 2025 15:19:09 +0200
From: Borislav Petkov <bp@...en8.de>
To: Uros Bizjak <ubizjak@...il.com>
Cc: X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH -tip] x86: Remove code depending on
 __GCC_ASM_FLAG_OUTPUTS__

On Fri, Sep 05, 2025 at 02:36:24PM +0200, Uros Bizjak wrote:
> There we are testing the value of a single flag bit, set by the CMPXCHG 
> instruction. =@ccz is used here to distinguish the CC user from comparison
> instructions, where ZERO flag indeed means that values are equal.

No, this is not what I mean. I mean SETE and SETZ are the same insn: 0F 94 /0

What you mean is that CMPXCHG8B modifies the zero flag, thus you want to use
ccz which denotes the flag while "set equal" doesn't correspond to a flag in
rFLAGS.

But then you need to say that in the commit message instead of me asking about
it.

> The same situation is with carry bit setting BT instructions, where we use
> =@ccc instead of =@....

Ditto.

Thx.

-- 
Regards/Gruss,
    Boris.

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