From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.9 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id E6488C4BA2D for ; Thu, 27 Feb 2020 12:48:25 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B95FF24695 for ; Thu, 27 Feb 2020 12:48:25 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="YroC3Tky" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729019AbgB0MsY (ORCPT ); Thu, 27 Feb 2020 07:48:24 -0500 Received: from us-smtp-delivery-1.mimecast.com ([205.139.110.120]:57187 "EHLO us-smtp-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1728986AbgB0MsY (ORCPT ); Thu, 27 Feb 2020 07:48:24 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1582807703; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=LQHQp+fMAomVybEW4Laeozki/FbJS39MJ2LdVVufDJ8=; b=YroC3TkyQ5QBIZaXtlU5B0IxRwa0djhiPlsN85NAFfXF0p34SZCGF8YYNEhwWx22lPgf+L JdZiya7azDJJfNr+jxPI9/RSf0yLnySYJTk1meWm8dsV8+Dd8CYbBFMJaAmKNUoxUqfKOq yB1ikCI9SfcWMcZVFxzU8yJM07ip2Tc= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-108-Ng1ii0SFOCO9RNdVM_7r6Q-1; Thu, 27 Feb 2020 07:48:18 -0500 X-MC-Unique: Ng1ii0SFOCO9RNdVM_7r6Q-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 1265F1005512; Thu, 27 Feb 2020 12:48:17 +0000 (UTC) Received: from gondolin (ovpn-117-2.ams2.redhat.com [10.36.117.2]) by smtp.corp.redhat.com (Postfix) with ESMTP id ED0EB1001B34; Thu, 27 Feb 2020 12:48:12 +0000 (UTC) Date: Thu, 27 Feb 2020 13:48:10 +0100 From: Cornelia Huck To: Christian Borntraeger Cc: Michael Mueller , frankja@linux.vnet.ibm.com, kvm@vger.kernel.org, david@redhat.com, thuth@redhat.com, linux-s390@vger.kernel.org Subject: Re: [PATCH] KVM: s390: introduce module parameter kvm.use_gisa Message-ID: <20200227134810.268fc3b5.cohuck@redhat.com> In-Reply-To: References: <20200227091031.102993-1-mimu@linux.ibm.com> Organization: Red Hat GmbH MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Thu, 27 Feb 2020 13:27:10 +0100 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. use_gisa vs disable_gisa is more a personal preference; I don't mind keeping it as use_gisa. > > 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. I don't think you should try to overload a debug knob like that; it's now simple enough, adding more code also adds to the potential for errors. > > > > --- > > arch/s390/kvm/kvm-s390.c | 8 +++++++- > > 1 file changed, 7 insertions(+), 1 deletion(-) > > > > diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c > > index d7ff30e45589..5c2081488024 100644 > > --- a/arch/s390/kvm/kvm-s390.c > > +++ b/arch/s390/kvm/kvm-s390.c > > @@ -184,6 +184,11 @@ static u8 halt_poll_max_steal = 10; > > module_param(halt_poll_max_steal, byte, 0644); > > MODULE_PARM_DESC(halt_poll_max_steal, "Maximum percentage of steal time to allow polling"); > > > > +/* if set to true, the GISA will be initialized and used if available */ > > +static bool use_gisa = true; > > +module_param(use_gisa, bool, 0644); > > +MODULE_PARM_DESC(use_gisa, "Use the GISA if the host supports it."); Especially as the description explicitly says "if the host supports it" -- that's good enough for a new knob. > > + > > /* > > * For now we handle at most 16 double words as this is what the s390 base > > * kernel handles and stores in the prefix page. If we ever need to go beyond > > @@ -2504,7 +2509,8 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type) > > kvm->arch.use_skf = sclp.has_skey; > > spin_lock_init(&kvm->arch.start_stop_lock); > > kvm_s390_vsie_init(kvm); > > - kvm_s390_gisa_init(kvm); > > + if (use_gisa) > > + kvm_s390_gisa_init(kvm); > > KVM_EVENT(3, "vm 0x%pK created by pid %u", kvm, current->pid); > > > > return 0; > > > Reviewed-by: Cornelia Huck