linux-kernel - Re: [PATCH 01/12] x86_64: memset

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Message-ID: <20181221211739.GD8441@uranus>
Date:   Sat, 22 Dec 2018 00:17:39 +0300
From:   Cyrill Gorcunov <gorcunov@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Igor Stoppa <igor.stoppa@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        igor.stoppa@...wei.com, Nadav Amit <nadav.amit@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Ahmed Soliman <ahmedsoliman@...a.vt.edu>,
        linux-integrity@...r.kernel.org,
        kernel-hardening@...ts.openwall.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/12] x86_64: memset_user()

On Fri, Dec 21, 2018 at 01:07:21PM -0800, Matthew Wilcox wrote:
> On Fri, Dec 21, 2018 at 11:46:16PM +0300, Cyrill Gorcunov wrote:
> > Cast to unsigned char is needed in any case. And as far as I remember
> > we've been using this multiplication trick for a really long time
> > in x86 land. I'm out of sources right now but it should be somewhere
> > in assembly libs.
> 
> x86 isn't the only CPU.  Some CPUs have slow multiplies but fast shifts.

This is x86-64 patch, not some generic code.

> Also loading 0x0101010101010101 into a register may be inefficient on
> some CPUs.

It is pretty efficient on x86-64. Moreover the self dependents as
a |= a << b is a source for data hazards inside cpu engine. Anyway
i'm not going to insist, just wanted to remind about such trick.
Up to you what to choose.