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

[George Dunlap <george.dunlap@citrix.com>]

On 03/02/16 18:39, Andrew Cooper wrote:
> On 03/02/16 18:21, George Dunlap wrote:
>> 2. It's not technically difficult to extend the number of servers
>> supported to something sensible, like 4 (using 4 different write_dm
>> p2m types)
> 
> While technically true, spare bits in the pagetable entries are at a
> premium, and steadily decreasing as Intel are introducing new features.
> 
> We have 16 current p2m types, and a finite upper bound of 6 bits of p2m
> type space, already with a push to reduce this number.
> 
> While introducing 1 new p2m type for this purpose might be an acceptable
> tradeoff, a using a p2m type per ioreq server is not IMO.

It is true that we don't have a ton of elbow room to grow at the moment.

But we actually already have a single p2m type -- mmio_write_dm -- that
as far as I know is only being used by XenGT.  We don't actually need to
add any new p2m types for my initial proposal.

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).

 -George