Re: [Xen-devel] page table question!

[tgh <tianguanhua@xxxxxxxxxx>]

hi
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"?
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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> > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-devel
> >
> >
> >
> >     
>
>
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>   

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