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

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Message-ID: <5c3c2a13-b397-6e9f-8772-a8aaa3f70771@citrix.com>
Date:   Tue, 14 Feb 2017 16:18:55 +0000
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
CC:     <linux-arch@...r.kernel.org>, Juergen Gross <jgross@...e.com>,
        Jeremy Fitzhardinge <jeremy@...p.org>, <x86@...nel.org>,
        <kvm@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Rusty Russell <rusty@...tcorp.com.au>,
        <linux-kernel@...r.kernel.org>,
        <virtualization@...ts.linux-foundation.org>,
        Chris Wright <chrisw@...s-sol.org>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, <xen-devel@...ts.xenproject.org>,
        Alok Kataria <akataria@...are.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [Xen-devel] [PATCH v2] x86/paravirt: Don't make
 vcpu_is_preempted() a callee-save function

On 14/02/17 14:46, Waiman Long wrote:
> On 02/14/2017 04:39 AM, Peter Zijlstra wrote:
>> On Mon, Feb 13, 2017 at 05:34:01PM -0500, Waiman Long wrote:
>>> It is the address of &steal_time that will exceed the 32-bit limit.
>> That seems extremely unlikely. That would mean we have more than 4G
>> worth of per-cpu variables declared in the kernel.
> I have some doubt about if the compiler is able to properly use
> RIP-relative addressing for this. Anyway, it seems like constraints
> aren't allowed for asm() when not in the function context, at least for
> the the compiler that I am using (4.8.5). So it is a moot point.

You can work the issue of not having parameters in a plain asm()
statement by using an asm-offset, stringizing it, and have C put the
string fragments back together.

"cmpb $0, " STR(STEAL_TIME_preempted) "(%rax);"

~Andrew