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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) Content-Language: en-US To: Sean Christopherson , Christian Borntraeger Cc: David Matlack , Jon Cargille , Jim Mattson , James Morse , Alexandru Elisei , Suzuki K Poulose , David Hildenbrand , Cornelia Huck , Claudio Imbrenda , Vitaly Kuznetsov , Wanpeng Li , Joerg Roedel , linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, linux-mips@vger.kernel.org, kvm@vger.kernel.org, kvm-ppc@vger.kernel.org, linux-kernel@vger.kernel.org, Jing Zhang , Marc Zyngier , Huacai Chen , Aleksandar Markovic , Paul Mackerras , Janosch Frank References: <20210925005528.1145584-1-seanjc@google.com> <03f2f5ab-e809-2ba5-bd98-3393c3b843d2@de.ibm.com> From: Paolo Bonzini In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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