linux-kernel - Re: [PATCH -final] x86/clear

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Date:   Wed, 6 Jul 2022 12:24:12 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Mark Hemment <markhemm@...glemail.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        patrice.chotard@...s.st.com, Mikulas Patocka <mpatocka@...hat.com>,
        Lukas Czerner <lczerner@...hat.com>,
        Christoph Hellwig <hch@....de>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Chuck Lever <chuck.lever@...cle.com>,
        Hugh Dickins <hughd@...gle.com>, patches@...ts.linux.dev,
        Linux-MM <linux-mm@...ck.org>, mm-commits@...r.kernel.org,
        Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH -final] x86/clear_user: Make it faster

On Tue, Jul 05, 2022 at 07:01:06PM +0200, Borislav Petkov wrote:

> +	asm volatile(
> +		"1:\n\t"
> +		ALTERNATIVE_3("rep stosb",
> +			      "call clear_user_erms",	  ALT_NOT(X86_FEATURE_FSRM),
> +			      "call clear_user_rep_good", ALT_NOT(X86_FEATURE_ERMS),
> +			      "call clear_user_original", ALT_NOT(X86_FEATURE_REP_GOOD))
> +		"2:\n"
> +	       _ASM_EXTABLE_UA(1b, 2b)
> +	       : "+&c" (size), "+&D" (addr), ASM_CALL_CONSTRAINT
> +	       : "a" (0)
> +		/* rep_good clobbers %rdx */
> +	       : "rdx");

"+c" and "+D" should be enough for 1 instruction assembly?