Re: [PATCH v4 10/14] migration: add support to migrate shared regions list

[Ashish Kalra]

On Fri, Sep 10, 2021 at 09:11:09AM +0000, Wang, Wei W wrote:
> On Friday, September 10, 2021 4:48 PM, Ashish Kalra wrote:
> > On Fri, Sep 10, 2021 at 07:54:10AM +0000, Wang, Wei W wrote:
> > There has been a long discussion on this implementation on KVM mailing list.
> > Tracking shared memory via a list of ranges instead of using bitmap is more
> > optimal. Most of the guest memory will be private and the unencrypted/shared
> > regions are basically ranges/intervals, so easy to implement and maintain 
> > using
> > lists.
> 
> OK. At which version did you discuss this or do you have a link? (I didn't 
> find it in v9 KVM patches)
> 

You can follow this email thread:
https://lore.kernel.org/kvm/20201211225542.GA30409@ashkalra_ubuntu_server/

> > A list will consume much less memory than a bitmap.
> > 
> > The bitmap will consume more memory as it will need to be sized as per guest
> > RAM size and will remain sparsely populated due to limited amount of
> > shared/unencrypted guest memory regions.
> 
> I also thought about this. It depends on the guest.
> I think "A list will consume much less memory" is true when we assume most of 
> guest pages are private pages.
> From design perspective, what if guest chooses to have most of its pages 
> being shared?
> Lists might consume much more memory than bitmaps in some cases, I think.
> (Probably I can check your previous discussions first)
> 

This will probably depend on the most common use case and scenario, i
think that most common use case will be a mostly encrypted guest.

Thanks,
Ashish