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

From: Christian Borntraeger
Date: Mon Sep 27 2021 - 11:17:57 EST

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@xxxxxxxxxx>
AuthorDate: Fri Apr 17 15:14:46 2020 -0700
Commit:     Paolo Bonzini<pbonzini@xxxxxxxxxx>
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

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.

* just make halt_poll_ns read-only.