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Normally scratch page is not allowed when a vm is operate under page
fault mode, i.e., in the existing codes, DRM_XE_VM_CREATE_FLAG_SCRATCH_PAGE
and DRM_XE_VM_CREATE_FLAG_FAULT_MODE are mutual exclusive. The reason
is fault mode relies on recoverable page to work, while scratch page
can mute recoverable page fault.
On xe2 and xe3, out of bound prefetch can cause page fault and further
system hang because xekmd can't resolve such page fault. SYCL and OCL
language runtime requires out of bound prefetch to be silently dropped
without causing any functional problem, thus the existing behavior
doesn't meet language runtime requirement.
At the same time, HW prefetching can cause page fault interrupt. Due to
page fault interrupt overhead (i.e., need Guc and KMD involved to fix
the page fault), HW prefetching can be slowed by many orders of magnitude.
Fix those problems by allowing scratch page under fault mode for xe2 and
xe3. With scratch page in place, HW prefetching could always hit scratch
page instead of causing interrupt.
A side effect is, scratch page could hide application program error.
Application out of bound accesses are hided by scratch page mapping,
instead of get reported to user.
v2: Refine commit message (Thomas)
v3: Move the scratch page flag check to after scratch page wa (Thomas)
v4: drop NEEDS_SCRATCH macro (matt)
Add a comment to DRM_XE_VM_CREATE_FLAG_SCRATCH_PAGE
Signed-off-by: Oak Zeng <oak.zeng@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Link: https://lore.kernel.org/r/20250403165328.2438690-4-oak.zeng@intel.com
Signed-off-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@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 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.