[RFC PATCH 13/13] Documentation: Document PowerPC kernel DEXCR interface

[Benjamin Gray <bgray@linux.ibm.com>]

Describe the DEXCR and document how to interact with it via the
prctl and sysctl interfaces.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
---
 Documentation/powerpc/dexcr.rst | 183 ++++++++++++++++++++++++++++++++
 Documentation/powerpc/index.rst |   1 +
 2 files changed, 184 insertions(+)
 create mode 100644 Documentation/powerpc/dexcr.rst

diff --git a/Documentation/powerpc/dexcr.rst b/Documentation/powerpc/dexcr.rst
new file mode 100644
index 000000000000..3c995f4b9fe0
--- /dev/null
+++ b/Documentation/powerpc/dexcr.rst
@@ -0,0 +1,183 @@
+==========================================
+DEXCR (Dynamic Execution Control Register)
+==========================================
+
+Overview
+========
+
+The DEXCR is a privileged special purpose register (SPR) introduced in
+PowerPC ISA 3.1B (Power10) that allows per-cpu control over several dynamic
+execution behaviours. These behaviours include speculation (e.g., indirect
+branch target prediction) and enabling return-oriented programming (ROP)
+protection instructions.
+
+The execution control is exposed in hardware as up to 32 bits ('aspects') in
+the DEXCR. Each aspect controls a certain behaviour, and can be set or cleared
+to enable/disable the aspect. There are several variants of the DEXCR for
+different purposes:
+
+DEXCR
+    A priviliged SPR that can control aspects for userspace and kernel space
+HDEXCR
+    A hypervisor-privileged SPR that can control aspects for the hypervisor and
+    enforce aspects for the kernel and userspace.
+UDEXCR
+    An optional ultravisor-privileged SPR that can control aspects for the ultravisor.
+
+Userspace can examine the current DEXCR state using a dedicated SPR that
+provides a non-privileged read-only view of the userspace DEXCR aspects.
+There is also an SPR that provides a read-only view of the hypervisor enforced
+aspects, which ORed with the userspace DEXCR view gives the effective DEXCR
+state for a process.
+
+
+User API
+========
+
+prctl()
+-------
+
+A process can control its own userspace DEXCR value using the
+``PR_PPC_GET_DEXCR`` and ``PR_PPC_SET_DEXCR`` pair of
+:manpage:`prctl(2)` commands. These calls have the form::
+
+    prctl(PR_PPC_GET_DEXCR, unsigned long aspect, 0, 0, 0);
+    prctl(PR_PPC_SET_DEXCR, unsigned long aspect, unsigned long flags, 0, 0);
+
+Where ``aspect`` (``arg1``) is a constant and ``flags`` (``arg2``) is a bifield.
+The possible aspect and flag values are as follows. Note there is no relation
+between aspect value and ``prctl()`` constant value.
+
+.. flat-table::
+   :header-rows: 1
+   :widths: 2 7 1
+
+   * - ``prctl()`` constant
+     - Aspect name
+     - Aspect bit
+
+   * - ``PR_PPC_DEXCR_SBHE``
+     - Speculative Branch Hint Enable (SBHE)
+     - 0
+
+   * - ``PR_PPC_DEXCR_IBRTPD``
+     - Indirect Branch Recurrent Target Prediction Disable (IBRTPD)
+     - 3
+
+   * - ``PR_PPC_DEXCR_SRAPD``
+     - Subroutine Return Address Prediction Disable (SRAPD)
+     - 4
+
+   * - ``PR_PPC_DEXCR_NPHIE``
+     - Non-Privileged Hash Instruction Enable (NPHIE)
+     - 5
+
+.. flat-table::
+   :header-rows: 1
+   :widths: 2 8
+
+   * - ``prctl()`` flag
+     - Meaning
+
+   * - ``PR_PPC_DEXCR_PRCTL``
+     - This aspect can be configured with ``prctl(PR_PPC_SET_DEXCR, ...)``
+
+   * - ``PR_PPC_DEXCR_SET_ASPECT``
+     - This aspect is set
+
+   * - ``PR_PPC_DEXCR_FORCE_SET_ASPECT``
+     - This aspect is set and cannot be undone. A subsequent
+       ``prctl(..., PR_PPC_DEXCR_CLEAR_ASPECT)`` will fail.
+
+   * - ``PR_PPC_DEXCR_CLEAR_ASPECT``
+     - This aspect is clear
+
+Note that
+
+* The ``*_SET_ASPECT`` / ``*_CLEAR_ASPECT`` refers to setting/clearing the bit in the DEXCR.
+  For example::
+
+      prctl(PR_PPC_SET_DEXCR, PR_PPC_DEXCR_IBRTPD, PR_PPC_DEXCR_SET_ASPECT, 0, 0);
+
+  will set the IBRTPD aspect bit in the DEXCR, causing indirect branch prediction
+  to be disabled.
+
+* The status returned by ``PR_PPC_GET_DEXCR`` does not include any alternative
+  config overrides. To see the true DEXCR state software should read the appropriate
+  SPRs directly.
+
+* A forced aspect will still report ``PR_PPC_DEXCR_PRCTL`` if it would
+  otherwise be editable.
+
+* The aspect state when starting a process is copied from the parent's
+  state on :manpage:`fork(2)` and :manpage:`execve(2)`. Aspects may also be set
+  or cleared by the kernel on process creation.
+
+Use ``PR_PPC_SET_DEXCR`` with one of ``PR_PPC_DEXCR_SET_ASPECT``,
+``PR_PPC_DEXCR_FORCE_SET_ASPECT``, or ``PR_PPC_DEXCR_CLEAR_ASPECT`` to edit a
+ given aspect.
+
+Common error codes for both getting and setting the DEXCR are as follows:
+
+.. flat-table::
+   :header-rows: 1
+   :widths: 2 8
+
+   * - Error
+     - Meaning
+
+   * - ``EINVAL``
+     - The DEXCR is not supported by the kernel.
+
+   * - ``ENODEV``
+     - The aspect is not recognised by the kernel or not supported by the hardware.
+
+``PR_PPC_SET_DEXCR`` may also report the following error codes:
+
+.. flat-table::
+   :header-rows: 1
+   :widths: 2 8
+
+   * - Error
+     - Meaning
+
+   * - ``ERANGE``
+     - ``arg2`` is incorrect. E.g., it does not select an action (set/clear),
+       or the flags are not recognised by the kernel.
+
+   * - ``ENXIO``
+     - The aspect is not editable via ``prctl()``.
+
+   * - ``EPERM``
+     - The process does not have sufficient privilege to modify this aspect,
+       or the aspect has been force set and cannot be modified.
+
+
+sysctl
+------
+
+Some aspects can be modified globally via :manpage:`sysctl(8)` entries. Such global
+modifications are applied after any process modifications. Any ``prctl()`` call to
+an overridden aspect this aspect may still report it as editable. The prctl setting
+will take effect again if the global override is restored to its default state.
+
+A global SBHE config is exposed in ``/proc/sys/kernel/speculative_branch_hint_enable``.
+Any process can read the current config value from it. Privileged processes can
+write to it to change the config. The new config is applied to all current and future
+processes (though note the kernel cannot override any hypervisor enforced aspects).
+
+.. flat-table::
+   :header-rows: 1
+   :widths: 2 8
+
+   * - Value
+     - Meaning
+
+   * - ``-1``
+     - Do not change from default or ``prctl()`` config.
+
+   * - ``0``
+     - Force clear aspect.
+
+   * - ``1``
+     - Force set aspect.
diff --git a/Documentation/powerpc/index.rst b/Documentation/powerpc/index.rst
index 85e80e30160b..d33b554ca7ba 100644
--- a/Documentation/powerpc/index.rst
+++ b/Documentation/powerpc/index.rst
@@ -15,6 +15,7 @@ powerpc
     cxl
     cxlflash
     dawr-power9
+    dexcr
     dscr
     eeh-pci-error-recovery
     elf_hwcaps
-- 
2.38.1