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Message-ID: <f37ab68c-61ce-b6fb-7a49-831bacfc7424@redhat.com>
Date:   Mon, 27 Sep 2021 17:03:55 +0200
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Subject: Re: disabling halt polling broken? (was Re: [PATCH 00/14] KVM:
 Halt-polling fixes, cleanups and a new stat)
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To:     Sean Christopherson <seanjc@google.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>
Cc:     David Matlack <dmatlack@google.com>,
        Jon Cargille <jcargill@google.com>,
        Jim Mattson <jmattson@google.com>,
        James Morse <james.morse@arm.com>,
        Alexandru Elisei <alexandru.elisei@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        David Hildenbrand <david@redhat.com>,
        Cornelia Huck <cohuck@redhat.com>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Joerg Roedel <joro@8bytes.org>,
        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 <jingzhangos@google.com>,
        Marc Zyngier <maz@kernel.org>,
        Huacai Chen <chenhuacai@kernel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>,
        Paul Mackerras <paulus@ozlabs.org>,
        Janosch Frank <frankja@linux.ibm.com>
References: <20210925005528.1145584-1-seanjc@google.com>
 <03f2f5ab-e809-2ba5-bd98-3393c3b843d2@de.ibm.com>
 <YVHcY6y1GmvGJnMg@google.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
In-Reply-To: <YVHcY6y1GmvGJnMg@google.com>
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On 27/09/21 16:59, Sean Christopherson wrote:
>> commit acd05785e48c01edb2c4f4d014d28478b5f19fb5
>> Author:     David Matlack<dmatlack@google.com>
>> AuthorDate: Fri Apr 17 15:14:46 2020 -0700
>> Commit:     Paolo Bonzini<pbonzini@redhat.com>
>> 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