Re: About releasing vcpu when closing vcpu fd

[Gu Zheng <guz.fnst@cn.fujitsu.com>]

Hi Gleb,

On 05/23/2014 05:43 PM, Gleb Natapov wrote:

> CCing Paolo.
> 
> On Tue, May 20, 2014 at 01:45:55PM +0800, Gu Zheng wrote:
>> Hi Gleb,
>> Excuse me for offline noisy.
> You will get much quicker response if you'll post to the list :)

Got it.:)

> 
>> There was a patch(from Chen Fan, last august) about releasing vcpu when
>> closing vcpu fd <http://www.spinics.net/lists/kvm/msg95701.html>, but
>> your comment said "Attempt where made to make it possible to destroy 
>> individual vcpus separately from destroying VM before, but they were
>> unsuccessful thus far."
>> So what is the pain here? If we want to achieve the goal, what should we do?
>> Looking forward to your further comments.:)
>>
> CPU array is accessed locklessly in a lot of places, so it will have to be RCUified.
> There was attempt to do so 2 year or so ago, but it didn't go anyware. Adding locks is
> to big a price to pay for ability to free a little bit of memory by destroying vcpu. 

Yes, it's a pain here. But if we want to implement "vcpu hot-remove", this must be
fixed sooner or later.
And any guys working on kvm "vcpu hot-remove" now?

> An
> alternative may be to make sure that stopped vcpu takes as little memory as possible.

Yeah. But if we add a new vcpu with the old id that we stopped before, it will fail.

Best regards,
Gu

> 
> --
> 			Gleb.
>