Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752323AbcKQFQu (ORCPT ); Thu, 17 Nov 2016 00:16:50 -0500 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:47718 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1750951AbcKQFQq (ORCPT ); Thu, 17 Nov 2016 00:16:46 -0500 Subject: Re: [PATCH v7 06/11] x86, paravirt: Add interface to support kvm/xen vcpu preempted check To: Peter Zijlstra 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> <20161116102355.GP3142@twins.programming.kicks-ass.net> 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 From: Pan Xinhui Date: Thu, 17 Nov 2016 13:16:30 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:45.0) Gecko/20100101 Thunderbird/45.4.0 MIME-Version: 1.0 In-Reply-To: <20161116102355.GP3142@twins.programming.kicks-ass.net> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 16111705-1617-0000-0000-0000017E27A0 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 16111705-1618-0000-0000-00004728910E Message-Id: X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2016-11-17_02:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=2 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1609300000 definitions=main-1611170098 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1072 Lines: 34 在 2016/11/16 18:23, Peter Zijlstra 写道: > 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. > yes, one more load in native case. I think this is acceptable as vcpu_is_preempted is not a critical function. however if we use pv_callee_save_regs_thunk, more unnecessary registers might be save/resotred in pv case. that will introduce a little overhead. but I think I am okay with your idea. I can make another patch based on this patchset with your suggested-by. thanks xinhui > Also, it generates more code. > > Paravirt muck should strive to be as cheap as possible when ran on > native hardware. >