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From: Thomas Gleixner <tglx@linutronix.de>
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To: Ammar Faizi <ammarfaizi2@gnuweeb.org>, Borislav Petkov <bp@alien8.de>
Cc: Ammar Faizi <ammarfaizi2@gnuweeb.org>, Alviro Iskandar Setiawan
 <alviro.iskandar@gnuweeb.org>, Dave Hansen <dave.hansen@linux.intel.com>,
 "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>, Tony
 Luck <tony.luck@intel.com>, Yazen Ghannam <yazen.ghannam@amd.com>, Linux
 Edac Mailing List <linux-edac@vger.kernel.org>, Linux Kernel Mailing List
 <linux-kernel@vger.kernel.org>, Stable Kernel <stable@vger.kernel.org>,
 GNU/Weeb Mailing List <gwml@vger.gnuweeb.org>, x86 Mailing List
 <x86@kernel.org>, David Laight <David.Laight@ACULAB.COM>, Jiri Hladky
 <hladky.jiri@googlemail.com>
Subject: Re: [PATCH v6 1/2] x86/delay: Fix the wrong asm constraint in
 `delay_loop()`
In-Reply-To: <87zgl2ksu3.ffs@tglx>
References: <20220329104705.65256-1-ammarfaizi2@gnuweeb.org>
 <20220329104705.65256-2-ammarfaizi2@gnuweeb.org> <87zgl2ksu3.ffs@tglx>
Date: Sun, 03 Apr 2022 19:14:34 +0200
Message-ID: <87tubaks1x.ffs@tglx>
MIME-Version: 1.0
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List-Id: <gwml.vger.gnuweeb.org>

On Sun, Apr 03 2022 at 18:57, Thomas Gleixner wrote:
> On Tue, Mar 29 2022 at 17:47, Ammar Faizi wrote:
>> The asm constraint does not reflect that the asm statement can modify
>> the value of @loops. But the asm statement in delay_loop() does modify
>> the @loops.
>>
>> Specifiying the wrong constraint may lead to undefined behavior, it may
>> clobber random stuff (e.g. local variable, important temporary value in
>> regs, etc.). This is especially dangerous when the compiler decides to
>> inline the function and since it doesn't know that the value gets
>> modified, it might decide to use it from a register directly without
>> reloading it.

Ignore me, I misread this part of the explanation.

Thanks,

        tglx