Re: More page-table questions.

[PUCCETTI Armand <armand.puccetti@cea.fr>]

Keir Fraser a écrit :
> On 12/3/07 16:26, "Petersson, Mats" <Mats.Petersson@amd.com> wrote:
>
>   
>> Are the page-tables ever updated directly by the guest, or is it all
>> done via hyper-calls?
>>     
>
> Leaf PTEs (i.e., really just PTEs, not PDEs) can be directly written from
> the point-of-view of the guest. In fact they are trapped and emulated by
> Xen. The guest is somewhat aware of this because it has explicitly
> write-protected all its pagetables, so if it were to attempt the direct
> write on native hardware in these circumstances it would receive a page
> fault.
>
>  -- Keir
>
>
>   
This is unclear to me: "a guest believes he can write PTEs" means that
his source code to access the page tables is left unchanged between 
legacy and PV version?

Merely, the hypervisor traps the guest's accesses to the page tables, to 
control
what he is doing (e.g. not overlapping any other domain's pages) and 
allowing or denying
any writes. This should apply to any page table level, so why only 
blocking writes to PTEs?

This is for 4K pages, but how are 2M pages mixed? or do we assume that 
every domain pages
are 4K?

Armand