[PATCH v8 02/15] genpt: Add Documentation/ files

Jason Gunthorpe
2025-11-04T18:30:20Z

Add some general description and pull in the kdoc comments from the source
file to index most of the useful functions.

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Reviewed-by: Samiullah Khawaja <skhawaja@google.com>
Tested-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
Tested-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
---
Documentation/driver-api/generic_pt.rst | 142 ++++++++++++++++++++++++
Documentation/driver-api/index.rst | 1 +
2 files changed, 143 insertions(+)
create mode 100644 Documentation/driver-api/generic_pt.rst

diff --git a/Documentation/driver-api/generic_pt.rst b/Documentation/driver-api/generic_pt.rst
new file mode 100644
index 00000000000000..210d1229aa1c1f
--- /dev/null
+++ b/Documentation/driver-api/generic_pt.rst
@@ -0,0 +1,142 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+========================
+Generic Radix Page Table
+========================
+
+.. kernel-doc:: include/linux/generic_pt/common.h
+ :doc: Generic Radix Page Table
+
+.. kernel-doc:: drivers/iommu/generic_pt/pt_defs.h
+ :doc: Generic Page Table Language
+
+-----
+Usage
+-----
+
+Generic PT is structured as a multi-compilation system. Since each format
+provides an API using a common set of names there can be only one format active
+within a compilation unit. This design avoids function pointers around the low
+level API.
+
+Instead the function pointers can end up at the higher level API (i.e.
+map/unmap, etc.) and the per-format code can be directly inlined into the
+per-format compilation unit. For something like IOMMU each format will be
+compiled into a per-format IOMMU operations kernel module.
+
+For this to work the .c file for each compilation unit will include both the
+format headers and the generic code for the implementation. For instance in an
+implementation compilation unit the headers would normally be included as
+follows::
+
+generic_pt/fmt/iommu_amdv1.c::
+
+ #include <linux/generic_pt/common.h>
+ #include "defs_amdv1.h"
+ #include "../pt_defs.h"
+ #include "amdv1.h"
+ #include "../pt_common.h"
+ #include "../pt_iter.h"
+ #include "../iommu_pt.h" /* The IOMMU implementation */
+
+iommu_pt.h includes definitions that will generate the operations functions for
+map/unmap/etc. using the definitions provided by AMDv1. The resulting module
+will have exported symbols named like pt_iommu_amdv1_init().
+
+Refer to drivers/iommu/generic_pt/fmt/iommu_template.h for an example of how the
+IOMMU implementation uses multi-compilation to generate per-format ops structs
+pointers.
+
+The format code is written so that the common names arise from #defines to
+distinct format specific names. This is intended to aid debuggability by
+avoiding symbol clashes across all the different formats.
+
+Exported symbols and other global names are mangled using a per-format string
+via the NS() helper macro.
+
+The format uses struct pt_common as the top-level struct for the table,
+and each format will have its own struct pt_xxx which embeds it to store
+format-specific information.
+
+The implementation will further wrap struct pt_common in its own top-level
+struct, such as struct pt_iommu_amdv1.
+
+----------------------------------------------
+Format functions at the struct pt_common level
+----------------------------------------------
+
+.. kernel-doc:: include/linux/generic_pt/common.h
+ :identifiers:
+.. kernel-doc:: drivers/iommu/generic_pt/pt_common.h
+
+-----------------
+Iteration Helpers
+-----------------
+
+.. kernel-doc:: drivers/iommu/generic_pt/pt_iter.h
+
+----------------
+Writing a Format
+----------------
+
+It is best to start from a simple format that is similar to the target. x86_64
+is usually a good reference for something simple, and AMDv1 is something fairly
+complete.
+
+The required inline functions need to be implemented in the format header.
+These should all follow the standard pattern of::
+
+ static inline pt_oaddr_t amdv1pt_entry_oa(const struct pt_state *pts)
+ {
+ [..]
+ }
+ #define pt_entry_oa amdv1pt_entry_oa
+
+where a uniquely named per-format inline function provides the implementation
+and a define maps it to the generic name. This is intended to make debug symbols
+work better. inline functions should always be used as the prototypes in
+pt_common.h will cause the compiler to validate the function signature to
+prevent errors.
+
+Review pt_fmt_defaults.h to understand some of the optional inlines.
+
+Once the format compiles then it should be run through the generic page table
+kunit test in kunit_generic_pt.h using kunit. For example::
+
+ $ tools/testing/kunit/kunit.py run --build_dir build_kunit_x86_64 --arch x86_64 --kunitconfig ./drivers/iommu/generic_pt/.kunitconfig amdv1_fmt_test.*
+ [...]
+ [11:15:08] Testing complete. Ran 9 tests: passed: 9
+ [11:15:09] Elapsed time: 3.137s total, 0.001s configuring, 2.368s building, 0.311s running
+
+The generic tests are intended to prove out the format functions and give
+clearer failures to speed up finding the problems. Once those pass then the
+entire kunit suite should be run.
+
+---------------------------
+IOMMU Invalidation Features
+---------------------------
+
+Invalidation is how the page table algorithms synchronize with a HW cache of the
+page table memory, typically called the TLB (or IOTLB for IOMMU cases).
+
+The TLB can store present PTEs, non-present PTEs and table pointers, depending
+on its design. Every HW has its own approach on how to describe what has changed
+to have changed items removed from the TLB.
+
+PT_FEAT_FLUSH_RANGE
+-------------------
+
+PT_FEAT_FLUSH_RANGE is the easiest scheme to understand. It tries to generate a
+single range invalidation for each operation, over-invalidating if there are
+gaps of VA that don't need invalidation. This trades off impacted VA for number
+of invalidation operations. It does not keep track of what is being invalidated;
+however, if pages have to be freed then page table pointers have to be cleaned
+from the walk cache. The range can start/end at any page boundary.
+
+PT_FEAT_FLUSH_RANGE_NO_GAPS
+---------------------------
+
+PT_FEAT_FLUSH_RANGE_NO_GAPS is similar to PT_FEAT_FLUSH_RANGE; however, it tries
+to minimize the amount of impacted VA by issuing extra flush operations. This is
+useful if the cost of processing VA is very high, for instance because a
+hypervisor is processing the page table with a shadowing algorithm.
diff --git a/Documentation/driver-api/index.rst b/Documentation/driver-api/index.rst
index 3e2a270bd82826..baff96b5cf0ba4 100644
--- a/Documentation/driver-api/index.rst
+++ b/Documentation/driver-api/index.rst
@@ -93,6 +93,7 @@ Subsystem-specific APIs
frame-buffer
aperture
generic-counter
+ generic_pt
gpio/index
hsi
hte/index
--
2.43.0