From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:13324 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729134AbgB0QJv (ORCPT ); Thu, 27 Feb 2020 11:09:51 -0500 Received: from pps.filterd (m0098399.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.42/8.16.0.42) with SMTP id 01RG4I5P043868 for ; Thu, 27 Feb 2020 11:09:50 -0500 Received: from e06smtp03.uk.ibm.com (e06smtp03.uk.ibm.com [195.75.94.99]) by mx0a-001b2d01.pphosted.com with ESMTP id 2ydkfapcbb-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Thu, 27 Feb 2020 11:09:48 -0500 Received: from localhost by e06smtp03.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Thu, 27 Feb 2020 16:09:46 -0000 Reply-To: mimu@linux.ibm.com Subject: Re: [PATCH] KVM: s390: introduce module parameter kvm.use_gisa References: <20200227091031.102993-1-mimu@linux.ibm.com> <48bbf704-11c7-0f67-a5ca-ae2841faccde@linux.ibm.com> <9195cc6d-a266-1c05-cba9-e434cd1bd0b7@de.ibm.com> From: Michael Mueller Date: Thu, 27 Feb 2020 17:10:45 +0100 MIME-Version: 1.0 In-Reply-To: <9195cc6d-a266-1c05-cba9-e434cd1bd0b7@de.ibm.com> Content-Type: text/plain; charset="utf-8"; format="flowed" Content-Language: en-US Content-Transfer-Encoding: 8bit Message-Id: <6d7b7e6b-23af-05c7-5b59-aa83f86ee2ad@linux.ibm.com> Sender: linux-s390-owner@vger.kernel.org List-ID: To: Christian Borntraeger , frankja@linux.vnet.ibm.com Cc: kvm@vger.kernel.org, cohuck@redhat.com, david@redhat.com, thuth@redhat.com, linux-s390@vger.kernel.org On 27.02.20 14:24, Christian Borntraeger wrote: > > > On 27.02.20 13:43, Michael Mueller wrote: >> >> >> On 27.02.20 13:27, Christian Borntraeger wrote: >>> >>> >>> On 27.02.20 10:10, Michael Mueller wrote: >>>> The boolean module parameter "kvm.use_gisa" controls if newly >>>> created guests will use the GISA facility if provided by the >>>> host system. The default is yes. >>>> >>>>    # cat /sys/module/kvm/parameters/use_gisa >>>>    Y >>>> >>>> The parameter can be changed on the fly. >>>> >>>>    # echo N > /sys/module/kvm/parameters/use_gisa >>>> >>>> Already running guests are not affected by this change. >>>> >>>> The kvm s390 debug feature shows if a guest is running with GISA. >>>> >>>>    # grep gisa /sys/kernel/debug/s390dbf/kvm-$pid/sprintf >>>>    00 01582725059:843303 3 - 08 00000000e119bc01  gisa 0x00000000c9ac2642 initialized >>>>    00 01582725059:903840 3 - 11 000000004391ee22  00[0000000000000000-0000000000000000]: AIV gisa format-1 enabled for cpu 000 >>>>    ... >>>>    00 01582725059:916847 3 - 08 0000000094fff572  gisa 0x00000000c9ac2642 cleared >>>> >>>> In general, that value should not be changed as the GISA facility >>>> enhances interruption delivery performance. >>>> >>>> A reason to switch the GISA facility off might be a performance >>>> comparison run or debugging. >>>> >>>> Signed-off-by: Michael Mueller >>> >>> Looks good to me. Regarding the other comments, I think allowing for dynamic changes >>> and keeping use_gisa vs disable_gisa makes sense. So I would think that the patch >>> as is makes sense. >>> >>> The only question is: shall we set use_gisa to 0 when the machine does not support >>> it (e.g. VSIE?) and then also forbid setting it to 1? Could be overkill. >> >> Then I would rename the parameter to "try_to_use_gisa" instead. (a joke ;) ) >> >> In that case we exit gisa_init() because of the missing AIV facility. >> >> void kvm_s390_gisa_init(struct kvm *kvm) >> { >>         struct kvm_s390_gisa_interrupt *gi = &kvm->arch.gisa_int; >> >> -->    if (!css_general_characteristics.aiv) >>                 return; >>         gi->origin = &kvm->arch.sie_page2->gisa; >>         gi->alert.mask = 0; >>     ... >> } >> > > I know. My point was more: "can we expose this". But this is probably overkill. I agree with Connie here, that would make the whole thing just more error-prone. That way the messages are at least consistent. >