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From: Sean Christopherson <seanjc@google.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>,
	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>
Subject: Re: disabling halt polling broken? (was Re: [PATCH 00/14] KVM: Halt-polling fixes, cleanups and a new stat)
Date: Mon, 27 Sep 2021 15:15:53 +0000	[thread overview]
Message-ID: <YVHgKWiU9WWL9ACg@google.com> (raw)
In-Reply-To: <f37ab68c-61ce-b6fb-7a49-831bacfc7424@redhat.com>

On Mon, Sep 27, 2021, Paolo Bonzini wrote:
> 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 (;;)".

Hmm, true, there is no danger to the system in having the capability override the
module param.

> 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

I think this option makes more sense, making halt_poll_ns read-only is basically
forcing users to switch to KVM_CAP_HALT_POLL.

> * just make halt_poll_ns read-only.
> 
> Paolo
>

  reply	other threads:[~2021-09-27 15:16 UTC|newest]

Thread overview: 50+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-25  0:55 [PATCH 00/14] KVM: Halt-polling fixes, cleanups and a new stat Sean Christopherson
2021-09-25  0:55 ` [PATCH 01/14] KVM: s390: Ensure kvm_arch_no_poll() is read once when blocking vCPU Sean Christopherson
2021-09-27  6:54   ` Christian Borntraeger
2021-09-25  0:55 ` [PATCH 02/14] KVM: Update halt-polling stats if and only if halt-polling was attempted Sean Christopherson
2021-09-28 18:57   ` David Matlack
2021-09-25  0:55 ` [PATCH 03/14] KVM: Refactor and document halt-polling stats update helper Sean Christopherson
2021-09-28 19:01   ` David Matlack
2021-09-25  0:55 ` [PATCH 04/14] KVM: Reconcile discrepancies in halt-polling stats Sean Christopherson
2021-09-28 21:26   ` David Matlack
2021-09-25  0:55 ` [PATCH 05/14] KVM: s390: Clear valid_wakeup in kvm_s390_handle_wait(), not in arch hook Sean Christopherson
2021-09-27  6:58   ` Christian Borntraeger
2021-09-25  0:55 ` [PATCH 06/14] KVM: Drop obsolete kvm_arch_vcpu_block_finish() Sean Christopherson
2021-09-27  6:58   ` Christian Borntraeger
2021-09-28 21:28   ` David Matlack
2021-09-25  0:55 ` [PATCH 07/14] KVM: Don't block+unblock when halt-polling is successful Sean Christopherson
2021-09-25  9:50   ` Marc Zyngier
2021-09-26  6:27     ` Paolo Bonzini
2021-09-26  9:02       ` Marc Zyngier
2021-09-27 17:28         ` Sean Christopherson
2021-09-28  9:24           ` Marc Zyngier
2021-09-28 16:21             ` Sean Christopherson
2021-09-30  9:36               ` Marc Zyngier
2021-09-25  0:55 ` [PATCH 08/14] KVM: x86: Tweak halt emulation helper names to free up kvm_vcpu_halt() Sean Christopherson
2021-09-28 21:59   ` David Matlack
2021-09-25  0:55 ` [PATCH 09/14] KVM: Rename kvm_vcpu_block() => kvm_vcpu_halt() Sean Christopherson
2021-09-27  7:06   ` Christian Borntraeger
2021-09-28 22:01   ` David Matlack
2021-09-25  0:55 ` [PATCH 10/14] KVM: Split out a kvm_vcpu_block() helper from kvm_vcpu_halt() Sean Christopherson
2021-09-27  7:41   ` Christian Borntraeger
2021-09-28 22:03   ` David Matlack
2021-09-25  0:55 ` [PATCH 11/14] KVM: stats: Add stat to detect if vcpu is currently blocking Sean Christopherson
2021-09-28 22:04   ` David Matlack
2021-09-25  0:55 ` [PATCH 12/14] KVM: Don't redo ktime_get() when calculating halt-polling stop/deadline Sean Christopherson
2021-09-28 22:08   ` David Matlack
2021-09-25  0:55 ` [PATCH 13/14] KVM: x86: Directly block (instead of "halting") UNINITIALIZED vCPUs Sean Christopherson
2021-09-28 22:12   ` David Matlack
2021-09-25  0:55 ` [PATCH 14/14] KVM: x86: Invoke kvm_vcpu_block() directly for non-HALTED wait states Sean Christopherson
2021-09-28 22:14   ` David Matlack
2021-09-27  7:22 ` disabling halt polling broken? (was Re: [PATCH 00/14] KVM: Halt-polling fixes, cleanups and a new stat) Christian Borntraeger
2021-09-27 14:59   ` Sean Christopherson
2021-09-27 15:03     ` Paolo Bonzini
2021-09-27 15:15       ` Sean Christopherson [this message]
2021-09-27 15:16       ` Christian Borntraeger
2021-09-27 16:58         ` David Matlack
2021-09-29  6:56           ` Christian Borntraeger
2021-09-27 17:24         ` Paolo Bonzini
2021-09-27 17:33           ` Sean Christopherson
2022-11-15  3:28             ` wangyanan (Y)
2022-11-16 17:19               ` David Matlack
2022-11-18  2:29                 ` wangyanan (Y)

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