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Date: Thu, 27 Aug 2020 11:55:05 -0700 From: "H. Peter Anvin" <hpa@...or.com> To: Uros Bizjak <ubizjak@...il.com>, x86@...nel.org, linux-kernel@...r.kernel.org Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de> Subject: Re: [PATCH v2] x86: Use xorl %0,%0 in __get_user_asm On 2020-08-27 11:09, Uros Bizjak wrote: > xorl %0,%0 is equivalent to xorq %0,%0 as both will zero the > entire register. Use xorl %0,%0 for all operand sizes to avoid > REX prefix byte when legacy registers are used and to avoid size > prefix byte when 16bit registers are used. > > Zeroing the full register is OK in this use case. xorl %0,%0 also > breaks register dependency chains, avoiding potential partial > register stalls with 8 and 16bit operands. > > The patch lowers the size of .fixup section by 20 bytes. > > Changes since v1: > - Rewrite commit message. > > Signed-off-by: Uros Bizjak <ubizjak@...il.com> > Cc: Thomas Gleixner <tglx@...utronix.de> > Cc: Ingo Molnar <mingo@...hat.com> > Cc: Borislav Petkov <bp@...en8.de> > Cc: "H. Peter Anvin" <hpa@...or.com> Reviewed-by: H. Peter Anvin (Intel) <hpa@...or.com>
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