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Historically, the Vertex Fetcher unit has not been an L3 client. That meant that, when a buffer containing vertex data was written to, it was necessary to issue a PIPE_CONTROL::VF Cache Invalidate to invalidate any VF L2 cachelines associated with that buffer, so the new value would be properly read from memory. Since Tigerlake and later, VERTEX_BUFFER_STATE and 3DSTATE_INDEX_BUFFER have included an "L3 Bypass Enable" bit which userspace drivers can set to request that the vertex fetcher unit snoop L3. However, unlike most true L3 clients, the "VF Cache Invalidate" bit continues to only invalidate the VF L2 cache - and not any associated L3 lines. To handle that, PIPE_CONTROL has a new "L3 Read Only Cache Invalidation Bit", which according to the docs, "controls the invalidation of the Geometry streams cached in L3 cache at the top of the pipe." In other words, the vertex and index buffer data that gets cached in L3 when "L3 Bypass Disable" is set. Mesa always sets L3 Bypass Disable so that the VF unit snoops L3, and whenever it issues a VF Cache Invalidate, it also issues a L3 Read Only Cache Invalidate so that both L2 and L3 vertex data is invalidated. xe is issuing VF cache invalidates too (which handles cases like CPU writes to a buffer between GPU batches). Because userspace may enable L3 snooping, it needs to issue an L3 Read Only Cache Invalidate as well. Fixes significant flickering in Firefox on Meteorlake, which was writing to vertex buffers via the CPU between batches; the missing L3 Read Only invalidates were causing the vertex fetcher to read stale data from L3. Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/4460 Fixes: |
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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 reStructuredText 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.