Re: question about memory mapping and balloon

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

Thank you for your reply


Mark Williamson 写道:
>>  In the paravirt case, guestos maintain its own mfn which need m2p and
>> p2m ,or xen manage these ?
>>     
>
> The guest is aware of it's mfns.
>
> The m2p table is a global machine frame -> pseudophysical frame 
> mapping.  Every guest uses this but is only privileged to update their 
> portions of this.  There's no requirement for it to be global but it 
> saves space this way.
>
> The p2m is maintained in the guest's private memory.
>   
I do not understand clearly why does xen maintain m2p table and why 
should the related portion of m2p be exported to the guestOS,and what 
does guestOS do by using m2p

>  
>>  I am confused about how does guestOS maintain its virt-to-physic and
>> physic-to-mach mapping ,in the linux ,there is only v2p mapping,
>>  how does guestOS maintain its p2m mapping ,
>>     
>
> It's given initial information about its memory layout at boot time.  
> It stores the p2m in a table and updates it when pages are transferred 
> (e.g. by the balloon driver or by page flipping in the network 
> device).  Xen's global M2P table is mapped into the guest and it keeps 
> this in sync by performing hypercalls to update it.
>   
when a VM boot with memsize=512M ,its p2m maybe only 64M or something 
,and when guest application request the memory,its virtual memory is 
mapped ,and when the application write some data into that memory,there 
is a page fault araised,and at that time guestOS update p2m and m2p and 
its page table(say virtual-to-machine address) ,is it right? or how do 
xen and guestOS deal with it?

could you help me
Thanks in advance

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