Re: [Xen-devel] Question about MMUEXT_PIN_L3_TABLE

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 09/07/2010 12:23, "W. Michael Petullo" <mike@xxxxxxxx> wrote:

>> The fourth L2 page directory (that is, the one pointed at by the fourth and
>> final L3 pgdir entry) must not be shared by multiple L3 page directories.
>> You need one such L2 pogdir for every L3 pgdir. Many OSes would normally
>> share these, and will need modifying to not do so.
> 
> As I understand this, I must create a new L2 page directory for my process
> and copy the entries in the corresponding kernel PD into it. Is this
> correct?

Yes.

>Could you provide some insigt into why this is required?

Xen owns private entries at the end of the fourth L2 page directory. Four of
these private entries map the contents of the L3 page directory entries
(yes, really) and obviously this arrangement only allows that fourth L2 page
directory to be mapped by one L3 page directory. Hence the unusual
restriction.

> Do
> I need to pin the L2 page directory before I pin the L3 page directory
> pointer table?

No. It gets implicitly pinned by the fact that it is mapped by a pinned L3.

 -- Keir



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