Does Xorg server pin composition buffer's MFNs?

Rafal Wojtczuk rafal at
Wed Apr 14 11:07:44 PDT 2010

A bit of background: apparently under Linux, the mfn (say, physical address) 
of the frame, that holds an usermode page, can change, unless precautions 
are made. See
why it might be an issue in certain virtualized environments.

Question 1: does X server (running under Linux), allocate the composition 
buffers in an usual way (using malloc() or similar), or does it do anything 
fancier in order to make sure the physical address of composition buffers is 
stable in time ? 
Question 2: Does ever X server schedule a DMA transaction (to graphics card 
presumably) from a composition buffer ? (in such case it would make sense to 
pin its physical pages).

Rafal Wojtczuk
The Qubes OS Project

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