disabling halt polling broken? (was Re: [PATCH 00/14] KVM: Halt-polling fixes, cleanups and a new stat)

[Christian Borntraeger]

Am 27.09.21 um 18:58 schrieb David Matlack:
> On Mon, Sep 27, 2021 at 8:17 AM Christian Borntraeger
> <borntraeger at de.ibm.com> wrote:
>>
>>
>>
>> Am 27.09.21 um 17:03 schrieb Paolo Bonzini:
>>> On 27/09/21 16:59, Sean Christopherson wrote:
>>>>> commit acd05785e48c01edb2c4f4d014d28478b5f19fb5
>>>>> Author:     David Matlack<dmatlack at google.com>
>>>>> AuthorDate: Fri Apr 17 15:14:46 2020 -0700
>>>>> Commit:     Paolo Bonzini<pbonzini at 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 (;;)".
> 
> I agree. It would also be a maintenance burden and subtle "gotcha" to
> have to increase halt_poll_ns anytime one wants to increase
> KVM_CAP_HALT_POLL.

I think the idea of the upper bound is not about preventing wasting CPUs
but to reconfigure existing poll intervals on a global level. So I think
this idea is a bad idea in itself. Especially as the admin might not have
access to the monitor of user QEMUs.

>>>
>>> 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
>>
>> what about using halt_poll_ns for those VMs that did not uses KVM_CAP_HALT_POLL and the private number for those that did.
> 
> None of these options would cover Christian's original use-case
> though. (Write to module to disable halt-polling system-wide.)
> 
> What about adding a writable "enable_halt_polling" module parameter

that would then affect both classes with and without KVM_CAP_HALT_POLL.

> that affects all VMs? Once that is in place we could also consider
> getting rid of halt_poll_ns entirely.

As far as I can tell QEMU does not yet use KVM_CAP_HALT_POLL.
So having a system wide halt_poll_ns makes sense. And I think for all
processes not using KVM_CAP_HALT_POLL we should really follow what
halt_poll_ns is NOW and not what it used to be.