RE: [Xen-devel] page table question!

["Petersson, Mats" <Mats.Petersson@xxxxxxx>]

> -----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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