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be75ece095
GPU capture bugs if heap sizes are not aligned to at least 16K. Ensuring that they are is not expected to impact memory usage since it seems the actual internal memory allocation is already aligned to 16K, the issue is only with how the heap reports its size versus the allocation size that capture uses. Reviewed-by: Aitor Camacho <aitor@lunarg.com> Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/41218>
288 lines
9.5 KiB
ReStructuredText
288 lines
9.5 KiB
ReStructuredText
KosmicKrisp workarounds
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#######################
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This file documents the relevant issues found in either Metal, the MSL
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compiler or any other component we have no control over that needed to be
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worked around to accomplish Vulkan conformance.
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All workarounds must be documented here and no code comment info should be
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provided other than the name ``KK_WORKAROUND_#``.
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Once a workaround was removed from the code, the code comment will be
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removed but the documentation here will be kept.
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Template
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========
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Use the following template to create documentation for a new workaround:
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.. code-block::
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KK_WORKAROUND_#
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---------------
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| macOS version:
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| Metal ticket:
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| Metal ticket status:
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| CTS test failure/crash:
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| Comments:
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| Log:
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``macOS version`` needs to have the OS version with which it was found.
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``Metal ticket`` needs to be either the Metal ticket number with the GitLab
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handle of the user that reported the ticket or ``Unreported``.
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``Metal ticket status`` needs to be either ``Fixed in macOS # (Build hash)``,
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``Waiting resolution`` or empty if unreported. If Apple reported that the issue
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was fixed, but no user has verified the fix, append ``[Untested]``.
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``CTS test failure/crash`` (remove ``failure`` or ``crash`` based on test
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behavior) needs to be the name of the test or test family the issue can be
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reproduced with.
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``Comments`` needs to include as much information on the issue and how the
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workaround fixes it.
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``Log`` needs to have the dates (yyyy-mm-dd, the only correct date format) with
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info on what was updated.
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Workarounds
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===========
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KK_WORKAROUND_9
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---------------
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| macOS version: 26.4.1
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| Metal ticket: Not reported
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| Metal ticket status:
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| CTS test failure: ``dEQP-VK.reconvergence.maximal.compute.nesting*``
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| Comments:
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Metal seems to re-order the sole break case of a loop such that execution
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reconverges earlier than expected.
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From the above mentioned CTS test, consider the following, which is the
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only code path that breaks within a loop:
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.. code-block:: c
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if (subgroupElect()) {
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outputC.loc[gl_LocalInvocationIndex]++;
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outputB.b[(outLoc++)*invocationStride + gl_LocalInvocationIndex] =
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subgroupBallot(true);
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break;
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}
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The test expects the ``subgroupBallot`` to yield just one bit set for the
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thread picked by ``subgroupElect``; however, Metal returns the full 0xFFFFFFFF,
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presumably because it re-ordered the operations to after the loop.
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To work around this, we add a trivial, always-true runtime condition to the
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break to ensure that the prior logic is not re-ordered.
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KK_WORKAROUND_8
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---------------
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| macOS version: 26.4.1
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| Metal ticket: FB22579201 (@squidbus)
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| Metal ticket status: Waiting resolution
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| CTS test failure: N/A
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| Comments:
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Metal GPU capture uses ``currentAllocatedSize`` to create an internal buffer
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over ``MTLHeap`` for the purpose of capturing its contents.
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Suppose we have a heap whose size is under the memory page size. Under native
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ARM execution, both the heap ``size`` and ``currentAllocatedSize`` will be
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aligned up to 16K. However, it has been observed that under Rosetta 2, ``size``
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will be aligned up to 4K but ``currentAllocatedSize`` will still be aligned up
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to 16K.
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These two in combination mean that, when GPU capture attempts to create buffers
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for these small heaps, it will fail, as ``currentAllocatedSize`` is larger than
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the heap ``size``. This will cause Metal validation layer errors if they are
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enabled, and attempting to take a GPU capture will crash the application.
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This workaround ensures that under Rosetta 2, heap sizes will be aligned to a
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minimum of 16K, prevening this scenario from occurring.
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| Log:
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| 2026-04-27: Workaround implemented
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KK_WORKAROUND_7
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---------------
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| macOS version: 26.0.1
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| Metal ticket: Not reported
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| Metal ticket status:
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| CTS test failure: ``dEQP-VK.renderpasses.renderpass2.depth_stencil_resolve.image_2d_*testing_stencil_samplemask``
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| Comments:
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Metal seems to ignore sample_mask out for cases for the stencil attachment
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where we have no color attachments, a multisample depth_stencil attachment
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with at least 2 samples and a fragment shader that only writes the depth and
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a static sample_mask out.
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My conclusion is that they may try to prematurely optimize by doing early
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fragment testing disregarding completely the sample_mask out and applying
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the value to all stencil samples.
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The failing tests do something along the lines of, start render pass with
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depth_stencil cleared to 0.0f and 0 respectively, if depth test passes set
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stencil to 1. Sample mask out is 1 (first sample). Draw framebuffer size square
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with 0.5f depth. End render pass storing values. Start render pass loading the
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previous output but if depth passes stencil will be set to 255. Sample mask out
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is 2 (second sample). Draw framebuffer size square with 0.5f depth.
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In a similar fashion to 2 workarounds below this one, we do a conditional
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discard at the end discarding fragments not covered by the coverage_mask.
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| Log:
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| 2026-02-05: Workaround implemented
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KK_WORKAROUND_6
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---------------
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| macOS version: 26.0.1
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| Metal ticket: Not reported
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| Metal ticket status:
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| CTS test failure: ``dEQP-VK.spirv_assembly.instruction.*.float16.opcompositeinsert.*``
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| Comments:
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Metal does not respect its own Memory Coherency Model (MSL spec 4.8). From
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the spec:
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``By default, memory in the device address space has threadgroup coherence.``
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If we have a single thread compute dispatch so that we do (simplified version):
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.. code-block:: c
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for (...) {
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value = ssbo_data[0]; // ssbo_data is a device buffer
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...
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ssbo_data[0] = new_value;
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}
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``ssbo_data[0]`` will not correctly store/load the values so the value
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written in iteration 0, will not be available in iteration 1. The workaround
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to this issue is marking the device memory pointer through which the memory
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is accessed as coherent so that the value is stored and loaded correctly.
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Hopefully this does not affect performance much.
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| Log:
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| 2025-12-08: Workaround implemented and reported to Apple
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KK_WORKAROUND_5
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---------------
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| macOS version: 26.0.1
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| Metal ticket: Not reported
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| Metal ticket status:
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| CTS test failure: ``dEQP-VK.fragment_operations.early_fragment.discard_no_early_fragment_tests_depth``
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| Comments:
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Fragment shaders that have side effects (like writing to a buffer) will be
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prematurely discarded if there is a ``discard_fragment`` that will always
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execute. To work around this, we just make the discard "optional" by moving
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it inside a run time conditional that will always be true (such as is the
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fragment a helper?). This tricks the MSL compiler into not optimizing it into
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a premature discard.
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| Log:
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| 2025-12-01: Workaround implemented
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KK_WORKAROUND_4
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---------------
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| macOS version: 26.0.1
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| Metal ticket: FB21124215 (@aitor)
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| Metal ticket status: Waiting resolution
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| CTS test failure: ``dEQP-VK.draw.renderpass.shader_invocation.helper_invocation*`` and few others
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| Comments:
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``simd_is_helper_thread()`` will always return true if the shader was started
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as a non-helper thread, even after ``discard_fragment()`` is called. The
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workaround is to have a variable tracking this state and update it when the
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fragment is discarded. This issue is present in M1 and M2 chips.
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| Log:
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| 2025-11-22: Workaround implemented and reported to Apple
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KK_WORKAROUND_3
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---------------
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| macOS version: 15.4.x
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| Metal ticket: FB20113490 (@aitor)
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| Metal ticket status: Waiting resolution
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| CTS test failure: ``dEQP-VK.subgroups.ballot_other.*.subgroupballotfindlsb``
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| Comments:
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``simd_is_first`` does not seem to behave as documented in the MSL
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specification. The following code snippet misbehaves:
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.. code-block:: c
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if (simd_is_first())
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temp = 3u;
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else
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temp = simd_ballot(true); /* <- This will return all active threads... */
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The way to fix this is by changing the conditional to:
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.. code-block:: c
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if (simd_is_first() && (ulong)simd_ballot(true))
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temp = 3u;
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else
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temp = (ulong)simd_ballot(true);
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| Log:
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| 2025-09-09: Workaround implemented and reported to Apple
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KK_WORKAROUND_2
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---------------
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| macOS version: 15.4.x
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| Metal ticket: FB21065475 (@aitor)
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| Metal ticket status: Waiting resolution
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| CTS test crash: ``dEQP-VK.graphicsfuzz.cov-nested-loops-never-change-array-element-one`` and ``dEQP-VK.graphicsfuzz.disc-and-add-in-func-in-loop``
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| Comments:
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We need to loop to infinite since MSL compiler crashes if we have something
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like (simplified version):
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.. code-block:: c
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while (true) {
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if (some_conditional) {
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break_loop = true;
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} else {
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break_loop = false;
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}
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if (break_loop) {
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break;
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}
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}
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The issue I believe is that ``some_conditional`` wouldn't change the value no
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matter in which iteration we are (something like fetching the same value from
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a buffer) and the MSL compiler doesn't seem to like that much to the point it
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crashes.
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The implemented solution is to change the ``while(true)`` loop with
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``for (uint64_t no_crash = 0u; no_crash < UINT64_MAX; ++no_crash)``, which
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tricks the MSL compiler into believing we are not doing an infinite loop
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(wink wink).
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| Log:
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| 2025-09-08: Workaround implemented
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KK_WORKAROUND_1
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---------------
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| macOS version: 15.4.x
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| Metal ticket: FB17604106 (@aitor)
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| Metal ticket status: [Untested] Fixed in macOS 26 Beta (25A5279m)
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| CTS test crash: ``dEQP-VK.glsl.indexing.tmp_array.vec3_dynamic_loop_write_dynamic_loop_read_fragment``
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| Comments:
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Uninitialized local scratch variable causes the MSL compiler to crash.
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Initialize scratch to avoid issue.
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| Log:
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| 2025-05-14: Workaround implemented and reported to Apple
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| 2025-06-14: Apple reported back saying it is now fixed in macOS 26 Beta (Build 25A5279m)
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