[v6,0/2] drm/i915: Failsafe migration blits

Message

Thomas Hellstrom Nov. 4, 2021, 11:07 a.m. UTC

This patch series introduces failsafe migration blits.
The reason for this seemingly strange concept is that if the initial
clearing or readback of LMEM fails for some reason[1], and we then set up
either GPU- or CPU ptes to the allocated LMEM, we can expose old
contents from other clients.

So after each migration blit to LMEM, attach a dma-fence callback that
checks the migration fence error value and if it's an error,
performs a memcpy blit, instead.

Patch 1 splits out the TTM move code into separate files
Patch 2 implements the failsafe blits and related self-tests

[1] There are at least two ways we could trigger exposure of uninitialized
LMEM assuming the migration blits themselves never trigger a gpu hang.

a) A gpu operation preceding a pipelined eviction blit resets and sets the
error fence to -EIO, and the error is propagated across the TTM manager to
the clear / swapin blit of a newly allocated TTM resource. It aborts and
leaves the memory uninitialized.

b) Something wedges the GT while a migration blit is submitted. It ends up
never executed and TTM can fault user-space cpu-ptes into uninitialized
memory.

v3:
- Style fixes in second patch (Matthew Auld)
v4:
- More style fixes in second patch (Matthew Auld)
v5:
- Fix an issue where we might end up waiting for a fence that would
  never signal.
v6:
- Fix a missing call to i915_ttm_memcpy_release() (Matthew Auld)

Thomas Hellström (2):
  drm/i915/ttm: Reorganize the ttm move code
  drm/i915/ttm: Failsafe migration blits

 drivers/gpu/drm/i915/Makefile                 |   1 +
 drivers/gpu/drm/i915/gem/i915_gem_ttm.c       | 328 ++---------
 drivers/gpu/drm/i915/gem/i915_gem_ttm.h       |  35 ++
 drivers/gpu/drm/i915/gem/i915_gem_ttm_move.c  | 523 ++++++++++++++++++
 drivers/gpu/drm/i915/gem/i915_gem_ttm_move.h  |  43 ++
 .../drm/i915/gem/selftests/i915_gem_migrate.c |  24 +-
 6 files changed, 673 insertions(+), 281 deletions(-)
 create mode 100644 drivers/gpu/drm/i915/gem/i915_gem_ttm_move.c
 create mode 100644 drivers/gpu/drm/i915/gem/i915_gem_ttm_move.h