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Re: [Xen-devel] [Question] How to support page offline in Xen environment

At 18:30 +0800 on 03 Dec (1228329001), Jiang, Yunhong wrote:
> a) A page is foreign mapped by another guest (e.g. dom0), change p2m entries 
> is not enough.


> b) A page is assigned to a domain with device assigned, we can't simply 
> change the p2m entry because of DMA operation may on-going. (this in fact 
> can't resolve cleanly through live migration, although the tools do hot 
> remove in advance).

That seems to be orthogonal to the question of how the page is got rid
of; you can do a hot remove and hot add whether you do a full
live-migrate or not.

> c) If a page is used like shadow page table or, virutal local apic's page, 
> currently we can't simply exchange these pages.

True, but live-migration doesn't help that because right now given an
MFN that's in use as a shadow page or any HVM state page you can't
easily find out which domain is responsible for it.

Also, remember that full live-migration needs enough spare RAM to hold
an extra copy of the guest, so it couldn't work on guests larger than
half the physical RAM, for example.

> d) For PV guest, can this be done without co-operation from guest?

Yes it can.  As long as you don't use the "fast path" resume to restart
the guest, it will re-read its p2m just like it would after a full

> Of course, we can simplify the request, for example, no support for page in 
> item a), b) and c) and that will be ok. 
> That's the reason we hope to get suggestion on next step.

I think it depends on how important it is to be able to offline frames
quickly and transparently.  If that's not important, then just save the
owning domain to disk, offline the frames, and restore it.  If it is
important, I'd be inclined to to something very lightweight based on
small parts of the save/restore code (which will be much faster than
live migration), and keep save-to-disk as a backstop for the edge cases.


Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Citrix Systems (R&D) Ltd.
[Company #02300071, SL9 0DZ, UK.]

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