Re: [Xen-devel] page table question!

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> I try to read the paging part code of xen-3.1.0, and I am confused about
> the " compat_machine_to_phys_mapping" ,what does xen do with the
> "CONFIG_COMPAT",and what are the differences between the
> "machine_to_phys_mapping" and the "compat_machine_to_phys_mapping"?

Compat-mode guests are 32-bit PAE guests running on a 64-bit hypervisor.  
Certain Xen API data structures need to be translated for compat-mode guests 
to access them; I guess the M2P mapping must be one of them but it's not 
immediately obvious to me what the differences would be.

For things like the start info / shared info page, which will also be 
different for compat-mode guests, it's more obvious since at the least there 
are probably some fields that are 64-bit or 32-bit depending on the guest 
mode...

Cheers,
Mark

> Could you help me
> Thanks in advance
>
> Petersson, Mats åé:
> >> -----Original Message-----
> >> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
> >> [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of MT Rezaie
> >> Sent: 13 June 2007 15:55
> >> To: xen-devel@xxxxxxxxxxxxxxxxxxx
> >> Subject: [Xen-devel] page table question!
> >>
> >> I want to know any difference between writing, reading and shadow page
> >> table (for example performance).
> >
> > Not sure exactly what you're asking for: are you asking about "writable"
> > and "non-writable" page-table vs. shadow-page-table?
> >
> > I believe "writable" page-table means that the guest is able to write
> > DIRECTLY to the lower level(s) of the page-table, and as such, we
> > "trust" the guest to do this correctly (and not map in stuff that
> > doesn't belong to the guest). I believe this mode isn't normally used by
> > Xen guests. This mode relies on the guest to correctly translate the
> > guest-physical address to a machine physical address.
> >
> > Non-writable page-tables are mapped "read-only" throughout the entire
> > page-table, and when a write occurs, the page-table is updated via the
> > emulation of the instruction in the hypervisor. The hypervisor can check
> > that the machine physical address is correct for the particular guest.
> >
> > As of currently, Shadow-mode is only used for HVM (fully virtualized)
> > domains. In this case, the guest has a "dummy" page-table that is not
> > used by the processor for any memory accesses, instead a "shadow" (copy)
> > of the guest's page-table is used. The shadow page-table is used by the
> > processor. To make updates to the page-table appear in the
> > shadow-page-table, the hypervisor is making sure that the
> > guest-page-table is read-only, and when the guest writes to the
> > page-table, we reproduce the write [with modification to reflect the
> > difference between guest physical and machine physical address].
> >
> > --
> > Mats
> >
> >> I am thankful for any help or resource for that.
> >>
> >> --
> >> Best Regards
> >> Mohamad Taghi Mir Mohamad Rezaie
> >>
> >> Iran University of Science and Technology
> >> Department of Computer Science & Engineering
> >> System Software Group
> >>
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> >
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> >
> >
> > .
>
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-- 
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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