On Wed, Apr 14, 2010 at 03:59:39PM -0400, Adam Jackson wrote: > On Wed, 2010-04-14 at 20:07 +0200, Rafal Wojtczuk wrote: > > Hello, > > 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 > > http://lists.xensource.com/archives/html/xen-devel/2010-04/msg00514.html > > 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). > > I'm assuming by "composition buffer" you mean the thing you're actually > scanning out on the display. No. I mean the per-window offscreen storage mechanism, activated by XCompositeRedirectSubwindows() function. Referred to as "windows backing pixmap" in http://ktown.kde.org/~fredrik/composite_howto.html. Apologies if I did not make it clear enough.
> I'll use the word "framebuffer" for that, since that's the usual X name for > it. > In the absence of an accelerated driver, X doesn't care about the > ... Unfortunately, that is not the piece of information I would like to know. In fact if CompositeRedirectManual flag is specified, the actual framebuffer is not touched by xlib calls at all. > It's not really clear what you're asking though, or what you're trying > to accomplish. Briefly, the goal is to get the location of a composition buffer created by X server running in virtual machine A, and map it in the address space of virtual machine B. Such mapping has to be defined in terms of physical addresses; consequently, it is crucial to make sure that the frames backing a composition buffer do not change in time. So, once again: is it true that X server allocates a composition buffer (a window backing pixmap) by a normal malloc call, and does no additional effort to pin its physical pages so that they do not change in time ? Regards, Rafal Wojtczuk The Qubes OS Project http://qubes-os.org _______________________________________________ [email protected]: X.Org support Archives: http://lists.freedesktop.org/archives/xorg Info: http://lists.freedesktop.org/mailman/listinfo/xorg
