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Date:	Tue, 10 Nov 2009 18:07:56 -0600
From:	Anthony Liguori <aliguori@...ibm.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	agl@...ux.vnet.ibm.com, qemu-devel@...gnu.org,
	Avi Kivity <avi@...hat.com>,
	virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: virtio: Add memory statistics reporting to the balloon driver

Rusty Russell wrote:
> On Wed, 11 Nov 2009 08:22:42 am Anthony Liguori wrote:
>   
>> Rusty Russell wrote:
>>     
>>> On Tue, 10 Nov 2009 03:02:06 am Adam Litke wrote:
>>>   
>>>       
>>>> A simpler approach is to collect memory statistics in the virtio
>>>> balloon driver and communicate them to the host via the device config space.
>>>>     
>>>>         
>>> There are two issues I see with this.  First, there's an atomicity problem
>>> since you can't tell when the stats are consistent.  Second, polling is
>>> ugly.
>>>
>>> A stats vq might solve this more cleanly?
>>>   
>>>       
>> This turns out to not work so nicely.  You really need bidirectional 
>> communication.  You need to request that stats be collected and then you 
>> need to tell the hypervisor about the stats that were collected.  You 
>> don't need any real correlation between requests and stat reports either.
>>     
>
> You register an outbuf at initialization time.  The host hands it back when
> it wants you to refill it with stats.
>   

That's strangely backwards.  Guest send a stat buffer that's filled out, 
host acks it when it wants another.  That doesn't seem bizarre to you?

>> This really models how target/actual work and I think it suggests that 
>> we want to reuse that mechanism for the stats too.
>>     
>
> Sure, I want to.  You want to.  It's simple.
>
> But the universe is remarkably indifferent to what we want.  Is it actually
> sufficient or are we going to regret our laziness?
>   

It's not laziness, it's consistency.  How is actual different than free 
memory or any other stat?

> Cheers,
> Rusty.
>   


-- 
Regards,

Anthony Liguori

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