Xen devel - Nov 2003 - And if guestOS doesn''t register a new page...

I''m currently reading the xen source (it''s very intresting and
cool :) and I
have a question concerning the memory management. To improve performance you
decided that the GuestOS is responsible for managing his hardware page table. It
just have to register its memory manipulations to Xen (using hypercall
HYPERVISOR_update_va_mapping() I think). 

At this point, guestOS relinquishes write privilege. This mechanism ensure that
a guestOS will not modify its page table for accessing memory owning by another
domain because Xen control that. So, the safety and isolation (concerning the
guestOS memory management) is allowed because guestOS registers its allocation
to the hypervisor. Thus, what happens if I build a guestOS which will not
register allocation? it could generate some problems? 

Best,
Guillaume    


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> I''m currently reading the xen source (it''s very
intresting and cool :) and I
> have a question concerning the memory management. To improve performance
you
> decided that the GuestOS is responsible for managing his hardware page
table. It
> just have to register its memory manipulations to Xen (using hypercall
> HYPERVISOR_update_va_mapping() I think). 
The fully general hypercall is
HYPERVISOR_mmu_update. update_va_mapping is a ''fast'' call that
is
useful when synchronously fixing up page faults.
> At this point, guestOS relinquishes write privilege. This mechanism ensure
that
> a guestOS will not modify its page table for accessing memory owning by
another
> domain because Xen control that. So, the safety and isolation (concerning
the
> guestOS memory management) is allowed because guestOS registers its
allocation
> to the hypervisor. Thus, what happens if I build a guestOS which will not
> register allocation? it could generate some problems? 
It cannot. This is because only page tables that have been registered
with Xen can ever be written to register CR3 (and therefore be used by
the real MMU). This is checked by Xen -- only Xen can change the
contents of CR3. See MMUEXT_NEW_BASEPTR in common/memory.c.

 -- Keir


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