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72ac304769
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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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.