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Date: Fri, 21 Dec 2018 23:05:46 +0300
From: Cyrill Gorcunov <gorcunov@gmail.com>
Subject: Re: [PATCH 01/12] x86_64: memset_user()
Message-ID: <20181221200546.GA8441@uranus>
References: <20181221181423.20455-1-igor.stoppa@huawei.com>
 <20181221181423.20455-2-igor.stoppa@huawei.com>
 <20181221182515.GF10600@bombadil.infradead.org>
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Sender: owner-linux-mm@kvack.org
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To: Matthew Wilcox <willy@infradead.org>
Cc: Igor Stoppa <igor.stoppa@gmail.com>, Andy Lutomirski <luto@amacapital.net>, Peter Zijlstra <peterz@infradead.org>, Dave Hansen <dave.hansen@linux.intel.com>, Mimi Zohar <zohar@linux.vnet.ibm.com>, Thiago Jung Bauermann <bauerman@linux.ibm.com>, igor.stoppa@huawei.com, Nadav Amit <nadav.amit@gmail.com>, Kees Cook <keescook@chromium.org>, Ahmed Soliman <ahmedsoliman@mena.vt.edu>, linux-integrity@vger.kernel.org, kernel-hardening@lists.openwall.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org

On Fri, Dec 21, 2018 at 10:25:16AM -0800, Matthew Wilcox wrote:
> On Fri, Dec 21, 2018 at 08:14:12PM +0200, Igor Stoppa wrote:
> > +unsigned long __memset_user(void __user *addr, int c, unsigned long size)
> > +{
> > +	long __d0;
> > +	unsigned long  pattern = 0;
> > +	int i;
> > +
> > +	for (i = 0; i < 8; i++)
> > +		pattern = (pattern << 8) | (0xFF & c);
> 
> That's inefficient.
> 
> 	pattern = (unsigned char)c;
> 	pattern |= pattern << 8;
> 	pattern |= pattern << 16;
> 	pattern |= pattern << 32;

Won't

	pattern = 0x0101010101010101 * c;

do the same but faster?