Comment 13 for bug 911236

In linus/master:

commit 5c0422878fcdc279ae9a8e8b66972a15b5efb67f
Author: Ben Widawsky <email address hidden>
Date: Mon Oct 17 15:51:55 2011 -0700

    drm/i915: ILK + VT-d workaround

    Idle the GPU before doing any unmaps. We know if VT-d is in use through
    an exported variable from iommu code.

    This should avoid a known HW issue.

    Signed-off-by: Ben Widawsky <email address hidden>
    Reviewed-by: Daniel Vetter <email address hidden>
    Signed-off-by: Keith Packard <email address hidden>

This does introduce a synchronous flush of the iommu tlb everytime a buffer is released which can cause stalls of ~1s every few seconds... But it does prevent the eventual machine hang. However, I would advise you to continue to use intel_iommu=igfx_off unless you truly do require virtualised graphics.