From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a
	callee-save function
Date: Mon, 13 Feb 2017 23:31:30 +0100
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Cc: linux-arch@vger.kernel.org, Juergen Gross <jgross@suse.com>,
	Jeremy Fitzhardinge <jeremy@goop.org>, x86@kernel.org, kvm@vger.kernel.org,
	Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
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	Paolo Bonzini <pbonzini@redhat.com>, linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.org,
	Chris Wright <chrisw@sous-sol.org>,
	Ingo Molnar <mingo@redhat.com>, hpa@zytor.com,
	xen-devel@lists.xenproject.org, Alok Kataria <akataria@vmware.com>,
	Thomas Gleixner <tglx@linutronix.de>
To: Waiman Long <longman@redhat.com>
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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.