From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:55878 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1729056AbgB0NYy (ORCPT ); Thu, 27 Feb 2020 08:24:54 -0500 Received: from pps.filterd (m0098416.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.42/8.16.0.42) with SMTP id 01RDKcRO098341 for ; Thu, 27 Feb 2020 08:24:53 -0500 Received: from e06smtp03.uk.ibm.com (e06smtp03.uk.ibm.com [195.75.94.99]) by mx0b-001b2d01.pphosted.com with ESMTP id 2yden2f7sq-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Thu, 27 Feb 2020 08:24:53 -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 13:24:51 -0000 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> From: Christian Borntraeger Date: Thu, 27 Feb 2020 14:24:46 +0100 MIME-Version: 1.0 In-Reply-To: <48bbf704-11c7-0f67-a5ca-ae2841faccde@linux.ibm.com> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 8bit Message-Id: <9195cc6d-a266-1c05-cba9-e434cd1bd0b7@de.ibm.com> Sender: linux-s390-owner@vger.kernel.org List-ID: To: mimu@linux.ibm.com, 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 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.