linux-kernel - Re: [PATCH v4] kvm: make vcpu life cycle separated from kvm instance

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Date:	Mon, 19 Dec 2011 10:16:39 +0900
From:	Takuya Yoshikawa <yoshikawa.takuya@....ntt.co.jp>
To:	Liu ping fan <kernelfans@...il.com>
CC:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, avi@...hat.com,
	aliguori@...ibm.com, gleb@...hat.com, mtosatti@...hat.com,
	jan.kiszka@....de
Subject: Re: [PATCH v4] kvm: make vcpu life cycle separated from kvm instance

Liu ping fan wrote:
> Suppose the following scene,
> Firstly, creating 10 kvm_vcpu for guest to take the advantage of
> multi-core. Now, reclaiming some of the kvm_vcpu, so we can limit the
> guest's usage of cpu. Then what about the kvm_vcpu unused? Currently
> they are just idle in kernel, but with this patch, we can remove them.

Then why not write it in the changelog?

>>> +void kvm_arch_vcpu_zap(struct work_struct *work)
>>> +{
>>> +     struct kvm_vcpu *vcpu = container_of(work, struct kvm_vcpu,
>>> +                     zap_work);
>>> +     struct kvm *kvm = vcpu->kvm;
>>>
>>> -     atomic_set(&kvm->online_vcpus, 0);
>>> -     mutex_unlock(&kvm->lock);
>>> +     kvm_clear_async_pf_completion_queue(vcpu);
>>> +     kvm_unload_vcpu_mmu(vcpu);
>>> +     kvm_arch_vcpu_free(vcpu);
>>> +     kvm_put_kvm(kvm);
>>>    }
>>
>> zap is really a good name for this?
>>
> zap = destroy, so I think it is OK.

Stronger than that.
My dictionary says "to destroy sth suddenly and with force."

In the case of shadow pages, I see what the author wanted to mean by "zap".

In your case, the host really destroy a VCPU suddenly?
The guest have to unplug it before, I guess.

If you just mean "destroy", why not use it?

>>> +#define kvm_for_each_vcpu(vcpu, kvm) \
>>> +     list_for_each_entry_rcu(vcpu,&kvm->vcpus, list)
>>
>> Is this macro really worth it?
>> _rcu shows readers important information, I think.
>>
> I guest kvm_for_each_vcpu is designed for hiding the details of
> internal implement, and currently it is implemented by array, and my
> patch will change it to linked-list,
> so IMO, we can still hide the details.

Then why are you doing
	list_add_rcu(&vcpu->list, &kvm->vcpus);
without introducing kvm_add_vcpu()?

You are just hiding part of the interface.
I believe this kind of incomplete abstraction should not be added.

The original code was complex enough to introduce a macro, but
list_for_each_entry_rcu(vcpu, &kvm->vcpus, list)
is simple and shows clear meaning by itself.

	Takuya
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