Re: [PATCH v4 1/2] x86/paravirt: Change vcp_is_preempted() arg type to long

From	Peter Zijlstra <peterz@infradead.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH v4 1/2] x86/paravirt: Change vcp_is_preempted() arg type to long
Date	2017-02-17 10:50 +0100
Message-ID	<tbNkS-2AC-25@gated-at.bofh.it> (permalink)
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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.

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[PATCH v4 0/2] x86/kvm: Reduce vcpu_is_preempted() overhead  Waiman Long <longman@redhat.com> - 2017-02-15 22:40 +0100
  [PATCH v4 2/2] x86/kvm: Provide optimized version of vcpu_is_preempted() for x86-64 Waiman Long <longman@redhat.com> - 2017-02-15 22:40 +0100
    Re: [PATCH v4 2/2] x86/kvm: Provide optimized version of  vcpu_is_preempted() for x86-64 Peter Zijlstra <peterz@infradead.org> - 2017-02-16 17:50 +0100
      Re: [PATCH v4 2/2] x86/kvm: Provide optimized version of  vcpu_is_preempted() for x86-64 Waiman Long <longman@redhat.com> - 2017-02-16 22:10 +0100
  [PATCH v4 1/2] x86/paravirt: Change vcp_is_preempted() arg type to long Waiman Long <longman@redhat.com> - 2017-02-15 22:40 +0100
    Re: [PATCH v4 1/2] x86/paravirt: Change vcp_is_preempted() arg type  to long Peter Zijlstra <peterz@infradead.org> - 2017-02-16 17:20 +0100
      Re: [PATCH v4 1/2] x86/paravirt: Change vcp_is_preempted() arg type  to long Waiman Long <longman@redhat.com> - 2017-02-16 22:10 +0100
        Re: [PATCH v4 1/2] x86/paravirt: Change vcp_is_preempted() arg type  to long Peter Zijlstra <peterz@infradead.org> - 2017-02-17 10:50 +0100

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