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

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Date:   Mon, 13 Feb 2017 12:06:44 -0800
From:   hpa@...or.com
To:     Peter Zijlstra <peterz@...radead.org>,
        Waiman Long <longman@...hat.com>
CC:     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 February 13, 2017 2:53:43 AM PST, Peter Zijlstra <peterz@...radead.org> wrote:
>On Mon, Feb 13, 2017 at 11:47:16AM +0100, Peter Zijlstra wrote:
>> That way we'd end up with something like:
>> 
>> asm("
>> push %rdi;
>> movslq %edi, %rdi;
>> movq __per_cpu_offset(,%rdi,8), %rax;
>> cmpb $0, %[offset](%rax);
>> setne %al;
>> pop %rdi;
>> " : : [offset] "i" (((unsigned long)&steal_time) + offsetof(struct
>steal_time, preempted)));
>> 
>> And if we could get rid of the sign extend on edi we could avoid all
>the
>> push-pop nonsense, but I'm not sure I see how to do that (then again,
>> this asm foo isn't my strongest point).
>
>Maybe:
>
>movsql %edi, %rax;
>movq __per_cpu_offset(,%rax,8), %rax;
>cmpb $0, %[offset](%rax);
>setne %al;
>
>?

We could kill the zero or sign extend by changing the calling interface to pass an unsigned long instead of an int.  It is much more likely that a zero extend is free for the caller than a sign extend.
-- 
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