Re: page table question!

[tgh <tianguanhua@ncic.ac.cn>]

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 写道:
>  
>
>   
>> -----Original Message-----
>> From: xen-devel-bounces@lists.xensource.com 
>> [mailto:xen-devel-bounces@lists.xensource.com] On Behalf Of MT Rezaie
>> Sent: 13 June 2007 15:55
>> To: xen-devel@lists.xensource.com
>> 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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