[RFC] PVFB: Add refresh period to XenStore parameters?

[Samuel Thibault <samuel.thibault@eu.citrix.com>]

Hello,

Sometimes the backend of PVFB knows that it doesn't need permanent
refresh, when the window is minimized for instance (no refresh at all),
or the administration tools know that the window is thumnailed, and so a
slow refresh rate is fine.  Also, some users may want to tune the
refresh rate according to the smoothness they would like, balanced with
the CPU time that requires.

I've played with that idea a bit and it seems to work fine, saving
computations and communications.  I'm now wondering about the interface:
it looks to me like it could be as simple as a "refresh-period" node in
the backend part of XenStore: the front-end would watch it, and update
the timing of its internal refresh loop, xenfb_fps in the case of Linux'
xenfb for instance.  A period of 0 would mean that no refresh is needed
(e.g. minimized window)

Samuel