linux-kernel - Re: [PATCH v4 1/2] x86/paravirt: Change vcp_is

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Date:   Fri, 17 Feb 2017 10:42:39 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     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>,
        "H. Peter Anvin" <hpa@...or.com>, 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 v4 1/2] x86/paravirt: Change vcp_is_preempted() arg type
 to long

On Thu, Feb 16, 2017 at 04:02:57PM -0500, Waiman Long wrote:
> On 02/16/2017 11:09 AM, Peter Zijlstra wrote:
> > On Wed, Feb 15, 2017 at 04:37:49PM -0500, Waiman Long wrote:
> >> The cpu argument in the function prototype of vcpu_is_preempted()
> >> is changed from int to long. That makes it easier to provide a better
> >> optimized assembly version of that function.
> >>
> >> For Xen, vcpu_is_preempted(long) calls xen_vcpu_stolen(int), the
> >> downcast from long to int is not a problem as vCPU number won't exceed
> >> 32 bits.
> >>
> > Note that because of the cast in PVOP_CALL_ARG1() this patch is
> > pointless.
> >
> > Then again, it doesn't seem to affect code generation, so why not. Takes
> > away the reliance on that weird cast.
> 
> I add this patch because I am a bit uneasy about clearing the upper 32
> bits of rdi and assuming that the compiler won't have a previous use of
> those bits. It gives me peace of mind.

So currently the PVOP_CALL_ARG#() macros force cast everything to
(unsigned long) anyway, but it would be good not to rely on that I
think, so yes.