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Date: Mon, 13 Feb 2017 23:31:30 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <longman@redhat.com>
Cc: hpa@zytor.com, Jeremy Fitzhardinge <jeremy@goop.org>,
        Chris Wright <chrisw@sous-sol.org>, Alok Kataria <akataria@vmware.com>,
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Subject: Re: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a
 callee-save function
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On Mon, Feb 13, 2017 at 05:24:36PM -0500, Waiman Long wrote:

> >> movsql %edi, %rax;
> >> movq __per_cpu_offset(,%rax,8), %rax;
> >> cmpb $0, %[offset](%rax);
> >> setne %al;

> I have thought of that too. However, the goal is to eliminate memory
> read/write from/to stack. Eliminating a register sign-extend instruction
> won't help much in term of performance.

Problem here is that all instructions have dependencies, so if you can
get rid of the sign extend mov you kill a bunch of stall cycles (I would
expect).

But yes, peanuts vs the stack load/stores.