From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from merlin.infradead.org (merlin.infradead.org [IPv6:2001:4978:20e::2]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 3tJgNJ5vbtzDvkx for ; Wed, 16 Nov 2016 21:24:16 +1100 (AEDT) Date: Wed, 16 Nov 2016 11:23:55 +0100 From: Peter Zijlstra To: Pan Xinhui Cc: Pan Xinhui , linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, virtualization@lists.linux-foundation.org, linux-s390@vger.kernel.org, xen-devel-request@lists.xenproject.org, kvm@vger.kernel.org, xen-devel@lists.xenproject.org, x86@kernel.org, benh@kernel.crashing.org, paulus@samba.org, mpe@ellerman.id.au, mingo@redhat.com, paulmck@linux.vnet.ibm.com, will.deacon@arm.com, kernellwp@gmail.com, jgross@suse.com, pbonzini@redhat.com, bsingharora@gmail.com, boqun.feng@gmail.com, borntraeger@de.ibm.com, rkrcmar@redhat.com, David.Laight@ACULAB.COM, dave@stgolabs.net, konrad.wilk@oracle.com Subject: Re: [PATCH v7 06/11] x86, paravirt: Add interface to support kvm/xen vcpu preempted check Message-ID: <20161116102355.GP3142@twins.programming.kicks-ass.net> References: <1478077718-37424-1-git-send-email-xinhui.pan@linux.vnet.ibm.com> <1478077718-37424-7-git-send-email-xinhui.pan@linux.vnet.ibm.com> <20161115154706.GF11311@worktop.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Wed, Nov 16, 2016 at 12:19:09PM +0800, Pan Xinhui wrote: > Hi, Peter. > I think we can avoid a function call in a simpler way. How about below > > static inline bool vcpu_is_preempted(int cpu) > { > /* only set in pv case*/ > if (pv_lock_ops.vcpu_is_preempted) > return pv_lock_ops.vcpu_is_preempted(cpu); > return false; > } That is still more expensive. It needs to do an actual load and makes it hard to predict the branch, you'd have to actually wait for the load to complete etc. Also, it generates more code. Paravirt muck should strive to be as cheap as possible when ran on native hardware.