Re: [Xen-devel] page table question!

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> In the HVM mode on the AMD NPT or intel EPT,VMM should maintain a
> counterpart pagetable for each of the guestOS process's page table, is
> it ? then there will not be a small mount of memory consumption for
> hyperviser's limited VA ,is it? or where are those counterpart page
> tables stored ?

As far as I know, the VMM only needs one extra table per guest, not per 
process.  This describes the "guest physical" -> "machine physical" mapping.  
Guest virtual addresses are then translated by walking the guest pagetable 
(guest virtual -> guest physical) and then the physical->machine table for 
that guest (maintained by the VMM) to get a real host address.

Cheers,
Mark

> Thanks in advance
>
> Tim Deegan åé:
> > At 17:35 +0100 on 13 Jun (1181756130), Mark Williamson wrote:
> >>>> One thing I've never been clear on for shadow mode is how
> >>>> accessed / dirty
> >>>> bits get propagated to the guest pagetable from the shadow.
> >>>
> >>> Good question. I have a feeling that the answer is "it doesn't". HAP
> >>> would probably solve this problem.
> >
> > When a guest pagetable entry has the Accessed bit clear, its shadow has
> > the Present bit clear.  This means we will get an extra page fault when
> > the guest tries to read that area of memory.  In the page-fault handler
> > we write the Accessed bit into the guest entry, and replace the shadow
> > entry with one that has the Present bit.  A similar scheme (shadowing
> > ~Dirty with ~Writeable) applies to those entries that have Dirty bits.
> >
> > The actual moving parts are in the _sh_propagate() function in
> > arch/x86/mm/shadow/multi.c, which is why that function needs to be told
> > whether it's handling a read fault, write fault or prefetch operation.
> >
> >> Don't guests need the dirty bits for their memory management (e.g. mmap)
> >> to work correctly?
> >
> > Yes, although in fact Xen doesn't quite catch all the cases where those
> > bits should be set (certain Xen-handled operations that walk the guest
> > pagetables instead of using the shadows) and it's not tripped us up
> > yet. :)
> >
> >> Maybe one could do something scary like trapping reads to
> >> guest PTEs, enabling Xen to refer to the real PTE...  Sounds a bit gross
> >> though.
> >
> > It was considered. :)  (For good reasons; talk to Michael Fetterman
> > some time.)
> >
> >> HAP is definitely HAPpier :-)
> >
> > Yep, should see a significant performance increase and eliminate a lot
> > of moving parts.
> >
> > Tim.



-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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