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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.
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