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When recording the default LRC, the expectation is that the hardware's
original state settings (both register and instruction) will be written
out to the LRC upon first context switch. For many 3DSTATE_* state
instructions that don't truly have "default" values, this translates to
a simple instruction header (opcodes + dword length) being written to
the LRC, followed by an appropriate number of blank dwords as a place
holder. When userspace creates a context (which starts as a copy of the
default LRC), they'll generally emit real 3DSTATE_* as part of their
initialization to select the settings they desire. If they don't emit
one of the 3DSTATE instructions, then the zeroed dwords that remain in
their LRC image generally translate to various state remaining disabled.
This will either be what userspace wants or will lead to very
reproducible and easily-debugged problems (rendering glitches, engine
hangs).
It turns out that a subset of the 3DSTATE instructions, specifically
those belonging to the SVG (State Variable - Global) unit, are not only
emitting 0's for the instruction's "body" dwords, but also for the
instruction header dword if no specific state has been explicitly set
before context switch. This means that when the hardware switches to a
context that hasn't explicitly provided an appropriate state setting,
the hardware will just see a sequence of NOOPs in the spot reserved for
that 3DSTATE instruction while executing the LRC, and the actual
hardware state setting will unintentionally inherit the configuration
used by the previously running context. Now when userspace makes a
mistake and forgets to emit an important state instruction they no
longer get consistent, easily-reproducible corruption/hangs, but rather
erratic behavior where the presence/absence of a problem depends on what
other workloads are running on the system and what order the contexts
are scheduled on the engine.
A specific example of this that came up recently related to mesh shading
The OpenGL driver was not specifically emitting a 3DSTATE_MESH_CONTROL
to disable mesh shading at context init, so on context switch, mesh
shading would either be on or off depending on what the previous context
had been doing. Vulkan apps _were_ enabling mesh shading, so running a
Vulkan app and then context switching to an OpenGL app resulted in mesh
shading still unexpectedly being enabled during OpenGL operation, and
since other Mesh-related state was not properly initialized for that
context a GPU hang was seen. Due to the specific ordering requirements
(Vulkan app runs first, followed by OpenGL app), it took additional
debug effort to track down the cause of the problem.
There are various workarounds related to this behavior, with current
implementations handled in the userspace drivers. E.g., Wa_14019789679
and Wa_22018402687. However it's been suggested that the kernel driver
can help simplify things here by emitting zeroed SVG state with proper
instruction headers as part of our default context creation (i.e., at
the same point we apply LRC workarounds). This will help ensure that
any future cases where a userspace driver does not emit an important
state setting will result in consistent behavior.
Bspec: 46261
Reviewed-by: Balasubramani Vivekanandan <balasubramani.vivekanandan@intel.com>
Link: https://lore.kernel.org/r/20231025151732.3461842-7-matthew.d.roper@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
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