Re: [kvm-devel] [RFC 0/8]KVM: swap out guest pages

[Rusty Russell]

On Tue, 2007-07-24 at 08:30 +0300, Avi Kivity wrote:
> Rusty Russell wrote:
> > On Mon, 2007-07-23 at 13:27 +0300, Avi Kivity wrote:
> >   
> >> Having an address_space (like your patch does) is remarkably simple, and 
> >> requires few hooks from the current vm.  However using existing vmas 
> >> mapped by the user has many advantages:
> >>
> >> - compatible with s390 requirements
> >> - allows the user to use hugetlbfs pages, which have a performance 
> >> advantage using ept/npt (but which are unswappable)
> >> - allows the user to map a file (which can be regarded as way to specify 
> >> the swap device)
> >> - better ingration with the rest of the vm
> >>     
> >
> > You don't need to expose the vmas.  You just have userspace point out
> > the start+len of each region of memory it wants the guest to be able to
> > access, and the address it wants it to appear in the guest.
> >
> > This is a slight superset of what lguest does in two ways:
> >
> > 1) my guest address == user address, but I'm looking at adding an offset
> > so I don't have to link the launcher binary specially.
> > 2) I have only one contiguous region of guest-physical memory, since I
> > can place device memory immediately above "normal" mem.
> >
> >   
> 
> My intent was to allow userspace to establish assign a virtual address
> range into a memory slot.
> 
> So long as you don't do swapping, all is simple, since you can do a
> get_user_pages() on initialization or when installing a shadow pte.  But
> if you want to swap, you need:
> 
> - a way to transfer the dirty bit from the shadow ptes to the struct page

Actually, get_user_pages() does that for you.  You have to make R/O any
writable pte where the guest doesn't set the dirty bit (so you can trap
it later) but last I put a printk in there, Linux doesn't do that.

> - a way to let the vm rmap know that there are shadow ptes that point to
> the page in addition to Linux ptes.  These shadow ptes may be in a
> different format than Linux ptes.
> - a different tlb invalidation method with ASIDs

Well first I was just going to see how well hooking into the shrinker
works.  That might be sufficient: just throw out shadow refs to pages
when there's pressure.

If not, it does get harder.  A callback in the mm struct to say "I want
to swap your page out" is required if we don't take a reference to the
page.  Dirty bit handling would be an interesting issue (maybe the
callback can say "No!" and dirty the page again?).

I fear mm code.

> It's not going to be simple.

Yeah, but it's one thing stopping lguest from being non-root usable, so
I want it there, too.

Cheers,
Rusty.


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