Date: Fri, 21 Dec 2018 23:46:16 +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 12:29:46PM -0800, Matthew Wilcox wrote: > > > > > > 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? > > Depends on your CPU. Some yes, some no. > > (Also you need to cast 'c' to unsigned char to avoid someone passing in > 0x1234 and getting 0x4646464646464634 instead of 0x3434343434343434) 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.
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