Re: [Xen-devel] Page tables in Xen

[tgh <wwwwww4187@xxxxxxxxxxx>]

hi
 in the PV mode,VM guestOS pagetable is virtual-to-machine address 
mapping,is it right?,and when a VM is migrated, the shadow page table 
mode will be enabled ,is it right? then, the PVguestOS pagetalbe will 
become the virtual-to-psuedophysical address mapping, or not?  then ,at 
that time ,the shadow pagetalbe is virtual-to-machine one ,is it right?
 I am still confused ,especially about the PVguestOS pagetalbe when 
shadow pagetable enabled for migration? is it a virtual-to-machine 
address mapping, or a virtual-to-psuedophysical address one ?
Thanks in advance







Mark Williamson åé:
> >   I am confused about shadow page table in the paravirtual mode in xen,
> >   does guestOS have all of its own processes'page tables,and  xen keeps
> > the very exactly all of the guestOS's processes page tables for a copy?
> > or say ,it is xen  who keeps the only copy of the guestOS's processes
> > page tables ,and guestOS  accesses to its processes's page tables
> > through some hypercall? or ,what about it ?
> >   If the guestOS has all of its processes's page talbes, while xen
> > maintain the copy of all those page tables ,is it an inefficient usage
> > of memory ,or what is the trick behind ?
> >     
>
> The process owns all of the pagetables for its processes; Xen does not (in 
> normal operation) maintain a shadow pagetable for PV domains.
> In PV mode, Xen simply prevents a guest from loading a pagetable for use 
> unless the guest has relinquished all rights to write to that memory.  i.e. a 
> guest can only register a pagetable with Xen if there's no way that the guest 
> can modify that pagetable directly.  This guarantees safety.
> When the guest needs to update the pagetable, it can either write directly to 
> it (which will cause a trap due to lack of permission, then Xen will check 
> whether the update should be allowed) or it can call directly to Xen (which 
> then does safety checks and does the update if it's OK).
> Cheers,
> Mark
>
>   
> > Thanks in advance
> >
> > Daniel Stodden åé:
> >     
> > > On Thu, 2007-11-29 at 11:00 +0530, pradeep singh rautela wrote:
> > >       
> > > > On Nov 29, 2007 1:42 AM, Haifeng He <hehaifeng2nd@xxxxxxxxx> wrote:
> > > >         
> > > > > Thanks for answering my question. I am talking about PV mode. What
> > > > > about shadow page table? Does it mean, Xen will keep (real) copies of
> > > > > page tables?
> > > > >           
> > > > Shadown page tables are employed in HVM guests not PV guests.
> > > >         
> > > regarding shadowed PV domains, there's still
> > > XENFEAT_auto_translated_physmap. but it's rather exotic and therefore
> > > rarely used.
> > >
> > > regarding the original question: yes, this means xen keeps the real page
> > > tables, while guests map pseudo-physical (i.e. linear) memory in what
> > > they (may) believe to be the real ones.
> > >
> > > regards,
> > > daniel
> > >       
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-devel
> >     
>
>
>   

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