Re: [Xen-devel] [PATCH v3 3/3] tools: introduce parameter max_wp_ram_ranges.

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Thu, Feb 4, 2016 at 8:51 AM, Yu, Zhang <yu.c.zhang@xxxxxxxxxxxxxxx> wrote:
>> Going forward, we probably will, at some point, need to implement a
>> parallel "p2t" structure to keep track of types -- and probably will
>> whether end up implementing 4 separate write_dm types or not (for the
>> reasons you describe).
>>
>
> Thank you, George. Could you please elaborate more about the idea of
> "p2t"?

So the p2m table is a partially-sparse structure designed to map pfns
to mfns.  At the bottom is a 64-bit entry that contains certain bits
specified by the hardware (mfn number, permissions, other features).
There are at the moment extra bits that aren't use, and in these bits
we store information about the pfn that Xen wants to know -- among
other things, the 'type' of the gpfn.

But as Andy pointed out, Intel are adding new features which take up
more of these bits; and at the same time, Xen has more features for
which using a p2m type is the most obvious solution.  So the idea
would be to have a separate structure, similar to the p2m table, but
wouldn't be used by the hardware -- it would only be used by Xen to
map pfn to type (or whatever other information it wanted about the
gpfn).  This does mean duplicating all the non-leaf nodes, as well as
doing two sparse-tree-walks rather  than just one when looking up gpfn
information.

 -George

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