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Date: Thu, 11 Jan 2018 10:47:50 -0800
From: Andi Kleen <andi@firstfloor.org>
To: Brian Gerst <brgerst@gmail.com>
Cc: Andi Kleen <andi@firstfloor.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        the arch/x86 maintainers <x86@kernel.org>,
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        Jiri Kosina <jikos@kernel.org>, Andi Kleen <ak@linux.intel.com>
Subject: Re: [PATCH v1 3/8] x86/entry/clearregs: Clear registers for 64bit
 SYSCALL
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On Wed, Jan 10, 2018 at 10:35:58PM -0500, Brian Gerst wrote:
> > @@ -263,6 +271,7 @@ entry_SYSCALL_64_fastpath:
> >  #endif
> >         ja      1f                              /* return -ENOSYS (already in pt_regs->ax) */
> >         movq    %r10, %rcx
> > +       xor     %r10, %r10
> 
> RCX is already clear, so xchgq %r10, %rcx will be simpler.

XOR is special cased by the hardware, so it's always more
efficient.

-Andi
>