Re: Swapping out kvm pages

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Chen Haogang wrote:
>
> In kvm's TODO page, I saw the item "attach kvm memory to a Linux 
> address_space so that guest memory can be paged out". But I don't 
> think it is as simple as this. Because both host part page fault and 
> guest part page fault interact.
>
> As I know, the creation of guest's shadow PTEs don't go through host's 
> PTEs. When a shadow PTE is to be created, it always assumes the page 
> pointer stored in memory region is vaild, and use the page struct to 
> calcuate host physical address.
>
> So, if we want to implement a swap mechanism. It is important to mark 
> the page struct pointer as invalid when host frame is swapped out. And 
> replace the page pointer with the new one when a frame is swapped in.
>
> Moreover, we should not swapped out pages that are currently mapped by 
> shadow PTEs unless we got a way to invalidate them. On the guest part, 
> when a shadow PTE is to be created, we should make sure the page 
> pointer in memory regeion is vaild ( i.e. the page is not swapped 
> out), if not, we should ask to host to bring in the page before guest 
> can resume.
>
> I want to know if I misunderstood anything, and wonder if there's any 
> easier way to implement guest pages' swapping?
>
>

That's what I had in mind.  I don't know exactly how you prevent a page 
from being swapped; maybe it involves the page locked page flag, or 
maybe just incrementing its reference count is enough.  Paging in is 
probably simpler; if the page isn't in the page cache, bring it in and 
attach it to the shadow page table.

The only complication I see is with the upcoming nested/extended page 
tables, which want shadow page tables for *all* guest pages, making this 
scheme fail.  We can make npt/ept demand paged like current shadow 
paging, but it seems to be a pessimization.


-- 
error compiling committee.c: too many arguments to function


-------------------------------------------------------------------------
Take Surveys. Earn Cash. Influence the Future of IT
Join SourceForge.net's Techsay panel and you'll get the chance to share your
opinions on IT & business topics through brief surveys-and earn cash
http://www.techsay.com/default.php?page=join.php&p=sourceforge&CID=DEVDEV