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([2001:b07:6468:f312:c8dd:75d4:99ab:290a]) by smtp.gmail.com with ESMTPSA id c9sm15544500wmb.41.2021.09.27.08.03.56 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 27 Sep 2021 08:03:57 -0700 (PDT) Message-ID: Date: Mon, 27 Sep 2021 17:03:55 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.1.0 Subject: Re: disabling halt polling broken? (was Re: [PATCH 00/14] KVM: Halt-polling fixes, cleanups and a new stat) To: Sean Christopherson , Christian Borntraeger References: <20210925005528.1145584-1-seanjc@google.com> <03f2f5ab-e809-2ba5-bd98-3393c3b843d2@de.ibm.com> From: Paolo Bonzini In-Reply-To: Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=pbonzini@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Cc: Wanpeng Li , kvm@vger.kernel.org, David Hildenbrand , linux-kernel@vger.kernel.org, Paul Mackerras , Claudio Imbrenda , kvmarm@lists.cs.columbia.edu, Janosch Frank , Marc Zyngier , Joerg Roedel , Huacai Chen , Aleksandar Markovic , Jon Cargille , kvm-ppc@vger.kernel.org, David Matlack , linux-arm-kernel@lists.infradead.org, Jim Mattson , Cornelia Huck , linux-mips@vger.kernel.org, Vitaly Kuznetsov X-BeenThere: kvmarm@lists.cs.columbia.edu X-Mailman-Version: 2.1.14 Precedence: list List-Id: Where KVM/ARM decisions are made List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Errors-To: kvmarm-bounces@lists.cs.columbia.edu Sender: kvmarm-bounces@lists.cs.columbia.edu On 27/09/21 16:59, Sean Christopherson wrote: >> commit acd05785e48c01edb2c4f4d014d28478b5f19fb5 >> Author: David Matlack >> AuthorDate: Fri Apr 17 15:14:46 2020 -0700 >> Commit: Paolo Bonzini >> CommitDate: Fri Apr 24 12:53:17 2020 -0400 >> >> kvm: add capability for halt polling >> >> broke the possibility for an admin to disable halt polling for already running KVM guests. >> In past times doing >> echo 0 > /sys/module/kvm/parameters/halt_poll_ns >> >> stopped polling system wide. >> Now all KVM guests will use the halt_poll_ns value that was active during >> startup - even those that do not use KVM_CAP_HALT_POLL. >> >> I guess this was not intended? No, but... > I would go so far as to say that halt_poll_ns should be a hard limit on > the capability ... this would not be a good idea I think. Anything that wants to do a lot of polling can just do "for (;;)". So I think there are two possibilities that makes sense: * track what is using KVM_CAP_HALT_POLL, and make writes to halt_poll_ns follow that * just make halt_poll_ns read-only. Paolo _______________________________________________ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm