On 27/09/21 16:59, Sean Christopherson wrote:

quoted

commit acd05785e48c01edb2c4f4d014d28478b5f19fb5
Author:     David Matlack[off-list ref]
AuthorDate: Fri Apr 17 15:14:46 2020 -0700
Commit:     Paolo Bonzini[off-list ref]
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


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