Re: [PATCH/RFC] KVM: track pid for VCPU only on KVM_RUN ioctl

Christian Borntraeger
2014-08-19T14:46:31Z

On 19/08/14 16:23, David Hildenbrand wrote:
>> On 19/08/14 14:14, David Hildenbrand wrote:
>>>> Il 19/08/2014 13:28, David Hildenbrand ha scritto:
>>>>> Looking at the code, kvm_cpu_synchronize_state() seems to do these ioctls in
>>>>> the vcpu thread (e.g. comming from cpu_synchronize_all_states()), any reasons
>>>>> why kvm_cpu_synchronize_post_reset() doesn't do the same (e.g. called from
>>>>> cpu_synchronize_all_post_reset())?
>>>>
>>>> No reason, feel free to post a patch for QEMU kvm-all.c.
>>>> Documentation/virtual/kvm/api.txt clearly says:
>>>>
>>>> Only run vcpu ioctls from the same thread that was used to create the
>>>> vcpu.
>>>>
>>>> Paolo
>>>>
>>>
>>> Thanks! A little more tweaking in the other parts of s390x resets
>>> and we should be able to reduce the number of "wrong" ioctls (I think I found
>>> most cases that are responsible for the performance degradation).
>>
>> Hmm. We want to not only reduce, we want them be zero.
>> In addition to a reworked MP_STATE patch set, we might be able to change the code to call "KVM_S390_INITIAL_RESET" only from the cpu thread itself.
>> If that simplifies things, we could avoid doing KVM_S390_INITIAL_RESET on CPU creation, because we know that all kernel version will do an implicit cpu reset on cpu creation anyway. Can you have a try on this as well when reworking that code? We could then fix this rcu performance penalty independent from getting rid of that ioctl.
>>
>> Christian
>>
>
> Already working on it, only one ioctl left on vcpu creation that is called
> from wrong context, trying to hide from me. Restarts and resets are already

Maybe its the synchronize when the oldpid is 0? Can you check the patch that I just sent?

> blasting fast.
>
> David
>