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

[Wanpeng Li <wanpeng.li@linux.intel.com>]

On Tue, Aug 19, 2014 at 04:04:03PM +0200, Christian Borntraeger wrote:
>On 18/08/14 07:02, Wanpeng Li wrote:
>> Hi Christian,
>> On Tue, Aug 05, 2014 at 04:44:14PM +0200, Christian Borntraeger wrote:
>>> We currently track the pid of the task that runs the VCPU in
>>> vcpu_load. Since we call vcpu_load for all kind of ioctls on a
>>> CPU, this causes hickups due to synchronize_rcu if one CPU is
>>> modified by another CPU or the main thread (e.g. initialization,
>>> reset). We track the pid only for the purpose of yielding, so
>>> let's update the pid only in the KVM_RUN ioctl.
>>>
>>> In addition, don't do a synchronize_rcu on startup (pid == 0).
>>>
>>> This speeds up guest boot time on s390 noticably for some configs, e.g.
>>> HZ=100, no full state tracking, 64 guest cpus 32 host cpus.
>>>
>>> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
>>> CC: Rik van Riel <riel@redhat.com>
>>> CC: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
>>> CC: Michael Mueller <mimu@linux.vnet.ibm.com>
>>> ---
>>> virt/kvm/kvm_main.c | 17 +++++++++--------
>>> 1 file changed, 9 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>>> index 9ae9135..ebc8f54 100644
>>> --- a/virt/kvm/kvm_main.c
>>> +++ b/virt/kvm/kvm_main.c
>>> @@ -124,14 +124,6 @@ int vcpu_load(struct kvm_vcpu *vcpu)
>>>
>>> 	if (mutex_lock_killable(&vcpu->mutex))
>>> 		return -EINTR;
>> 
>> One question: 
>> 
>>> -	if (unlikely(vcpu->pid != current->pids[PIDTYPE_PID].pid)) {
>> 
>> When vcpu->pid and current->pids[PIDTYPE_PID].pid will be different?
>
>If two different thread call an ioctl on a vcpu fd. (It must be an ioctl that has done vcpu_load - almost all except for some interrupt injections)

Thanks for your explanation. When can this happen?

Regards,
Wanpeng Li