When using an IOMMU, GEM objects are mapped by their DMA address as the
physical address is unknown. This depends on the underlying IOMMU
driver to map and unmap the physical pages properly as defined in
intel_iommu.c.
The current code will tell the IOMMU to unmap the GEM BO's pages on the
destruction of the first VMA that "maps" that BO. This is clearly wrong
as there may be other VMAs "mapping" that BO (using flink). The scanout
is one such example.
The patch fixes this issue by only unmapping the DMA maps when there are
no more VMAs mapping that object. This is equivalent to when an object
is considered unbound as can be seen by the code. On the first VMA that
again because bound, we will remap.
An alternate solution would be to move the dma mapping to object
creation and destrubtion. I am not sure if this is considered an
unfriendly thing to do.
Some notes to backporters trying to backport full PPGTT:
The bug can never be hit without enabling the IOMMU. The existing code
will also do the right thing when the object is shared via dmabuf. The
failure should be demonstrable with flink. In cases when not using
intel_iommu_strict it is likely (likely, as defined by: off the top of
my head) on current workloads to *not* hit this bug since we often
teardown all VMAs for an object shared across multiple VMs. We also
finish access to that object before the first dma_unmapping.
intel_iommu_strict with flinked buffers is likely to hit this issue.
Signed-off-by: Armin Reese <armin.c.reese@intel.com>
[danvet: Add the excellent commit message provided by Ben.]
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
************************************************************
* For the very latest on DRI development, please see: *
* http://dri.freedesktop.org/ *
************************************************************
The Direct Rendering Manager (drm) is a device-independent kernel-level
device driver that provides support for the XFree86 Direct Rendering
Infrastructure (DRI).
The DRM supports the Direct Rendering Infrastructure (DRI) in four major
ways:
1. The DRM provides synchronized access to the graphics hardware via
the use of an optimized two-tiered lock.
2. The DRM enforces the DRI security policy for access to the graphics
hardware by only allowing authenticated X11 clients access to
restricted regions of memory.
3. The DRM provides a generic DMA engine, complete with multiple
queues and the ability to detect the need for an OpenGL context
switch.
4. The DRM is extensible via the use of small device-specific modules
that rely extensively on the API exported by the DRM module.
Documentation on the DRI is available from:
http://dri.freedesktop.org/wiki/Documentation
http://sourceforge.net/project/showfiles.php?group_id=387
http://dri.sourceforge.net/doc/
For specific information about kernel-level support, see:
The Direct Rendering Manager, Kernel Support for the Direct Rendering
Infrastructure
http://dri.sourceforge.net/doc/drm_low_level.html
Hardware Locking for the Direct Rendering Infrastructure
http://dri.sourceforge.net/doc/hardware_locking_low_level.html
A Security Analysis of the Direct Rendering Infrastructure
http://dri.sourceforge.net/doc/security_low_level.html
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