Re: The mfn of the frame, that holds a mlock-ed PV domU usermode page, can change

[Jeremy Fitzhardinge <jeremy@goop.org>]

On 04/12/2010 02:19 PM, Joanna Rutkowska wrote:
> Right, that's for the "ring" page, which we use to implement a ring
> buffer, and we then pass mfns of the actual Xorg's composition buffers
> over this ring buffer to Dom0.
>
> Interestingly, I have never seen a garbage in any of the composition
> buffers (which are directly displayed by our appviewers, so it would be
> immediately visible), just like if only the mfn for the "ring" page
> could be modified, but the composition buffer's mfn were somehow pinned...
>
> This might suggest that the memory used by the composition buffers
> (which are in usermode) is somehow locked?
>   

Worth looking into.

I'm not at all familiar with how X manages composition buffers, but it
seems to me that in normal use, one would want to be able to either
allocate that buffer in texture memory (so it can be used as a texture
source), or at least copy updates into texture memory.  Couldn't you
hook into that transfer to the composition hardware (ie, dom0)?

    J