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xen-devel
Re: [Xen-devel] Page tables in Xen
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 写道:
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 写道:
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
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