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Date: Wed, 6 Jul 2022 12:24:12 +0300
From: Alexey Dobriyan <adobriyan@gmail.com>
To: linux-kernel@vger.kernel.org
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Mark Hemment <markhemm@googlemail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	the arch/x86 maintainers <x86@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>, patrice.chotard@foss.st.com,
	Mikulas Patocka <mpatocka@redhat.com>,
	Lukas Czerner <lczerner@redhat.com>, Christoph Hellwig <hch@lst.de>,
	"Darrick J. Wong" <djwong@kernel.org>,
	Chuck Lever <chuck.lever@oracle.com>,
	Hugh Dickins <hughd@google.com>, patches@lists.linux.dev,
	Linux-MM <linux-mm@kvack.org>, mm-commits@vger.kernel.org,
	Mel Gorman <mgorman@suse.de>
Subject: Re: [PATCH -final] x86/clear_user: Make it faster
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In-Reply-To: <YsRuUl24zkhpE3s/@zn.tnic>

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?