linux-kernel - Re: [PATCH v2] x86/paravirt: Don't make vcpu_is

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Date:   Mon, 13 Feb 2017 23:31:30 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     hpa@...or.com, Jeremy Fitzhardinge <jeremy@...p.org>,
        Chris Wright <chrisw@...s-sol.org>,
        Alok Kataria <akataria@...are.com>,
        Rusty Russell <rusty@...tcorp.com.au>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-arch@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        xen-devel@...ts.xenproject.org, kvm@...r.kernel.org,
        Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>
Subject: Re: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a
 callee-save function

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.