Re: [Qemu-devel] [PATCH 7/7] Fake dirty loggin when it's not there

[Stefano Stabellini]

On Fri, 17 Jul 2009, Alexander Graf wrote:
> Thinking about this again, you'd have to copy it somewhere before. So  
> on every dirty check you'd have to copy all memory off to "somewhere"  
> to be able to check for it again later.
> 
> Sounds feasible, but is it really worth the effort? Can Xen do dirty  
> logging?

Sure it can now, but it used to have a memcmp based mechanism before,
and it had decent performances, while setting everything as dirty every
time it is going to be a killer (at least if you do this under xen it is).

Besides I don't know how the dirty tracking is implemented in kvm, but
it is not so good from a performance point of view in the HAP case,
because you have to set the videoram as read only and catch every write
(this how we do it at least) so it would be interesting to compare the
two approaches.