Re: [PATCH 2/2] kvm: change the dirty page tracking to work with dirty bity

From: Marcelo Tosatti <mtosatti@redhat.com>
To: Izik Eidus <ieidus@redhat.com>
Cc: kvm@vger.kernel.org, avi@redhat.com
Subject: Re: [PATCH 2/2] kvm: change the dirty page tracking to work with dirty bity
Date: Thu, 11 Jun 2009 09:24:11 -0300	[thread overview]
Message-ID: <20090611122411.GB3944@amt.cnet> (raw)
In-Reply-To: <4A30EA32.7010000@redhat.com>

On Thu, Jun 11, 2009 at 02:27:46PM +0300, Izik Eidus wrote:
> Marcelo Tosatti wrote:
>>
>>
>> What i'm saying is with shadow and NPT (i believe) you can mark a spte
>> writable but not dirty, which gives you the ability to know whether
>> certain pages have been dirtied.
>>   
>
> Isnt this what this patch is doing?

Yes, was confused for some reason i don't remember.

So making the dirty bit available to the host is a good idea, but would
have to check things like faults on out of sync pagetables (where
the guest dirty bit might be cleared in parallel, maybe its ok but
not sure), verify transfer of dirty bit when zapping is consistent
everywhere, etc.

So it would be nicer to introduce an optimization to the way dirty bit
info is acquired, then you use that to optimize kvm's dirty log ioctl.

The link with KSM was that you can consult this dirty info, which is
fast, to know if content of pages has changed. But it maybe useless,
don't know.