@@ -36,8 +36,145 @@ state for a process.
Configuration
=============
-The DEXCR is currently unconfigurable. All threads are run with the
-NPHIE aspect enabled.
+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 which, 0, 0, 0);
+ prctl(PR_PPC_SET_DEXCR, unsigned long which, unsigned long ctrl, 0, 0);
+
+The possible 'which' and 'ctrl' values are as follows. Note there is no relation
+between the 'which' value and the DEXCR aspect's index.
+
+.. flat-table::
+ :header-rows: 1
+ :widths: 2 7 1
+
+ * - ``prctl()`` which
+ - Aspect name
+ - Aspect index
+
+ * - ``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()`` ctrl
+ - Meaning
+
+ * - ``PR_PPC_DEXCR_CTRL_EDITABLE``
+ - This aspect can be configured with PR_PPC_SET_DEXCR (get only)
+
+ * - ``PR_PPC_DEXCR_CTRL_SET``
+ - This aspect is set / set this aspect
+
+ * - ``PR_PPC_DEXCR_CTRL_CLEAR``
+ - This aspect is clear / clear this aspect
+
+ * - ``PR_PPC_DEXCR_CTRL_SET_ONEXEC``
+ - This aspect will be set after exec / set this aspect after exec
+
+ * - ``PR_PPC_DEXCR_CTRL_CLEAR_ONEXEC``
+ - This aspect will be clear after exec / clear this aspect after exec
+
+Note that
+
+* which is a plain value, not a bitmask. Aspects must be worked with individually.
+
+* ctrl is a bitmask. ``PR_PPC_GET_DEXCR`` returns both the current and onexec
+ configuration. For example, ``PR_PPC_GET_DEXCR`` may return
+ ``PR_PPC_DEXCR_CTRL_EDITABLE | PR_PPC_DEXCR_CTRL_SET |
+ PR_PPC_DEXCR_CTRL_CLEAR_ONEXEC``. This would indicate the aspect is currently
+ set, it will be cleared when you run exec, and you can change this with the
+ ``PR_PPC_SET_DEXCR`` prctl.
+
+* The set/clear terminology refers to setting/clearing the bit in the DEXCR.
+ For example::
+
+ prctl(PR_PPC_SET_DEXCR, PR_PPC_DEXCR_IBRTPD, PR_PPC_DEXCR_CTRL_SET, 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`` represents what value the process
+ would like applied. It does not include any alternative overrides, such as if
+ the hypervisor is enforcing the aspect be set. To see the true DEXCR state
+ software should read the appropriate SPRs directly.
+
+* The aspect state when starting a process is copied from the parent's state on
+ :manpage:`fork(2)`. The state is reset to a fixed value on
+ :manpage:`execve(2)`. The PR_PPC_SET_DEXCR prctl() can control both of these
+ values.
+
+* The ``*_ONEXEC`` controls do not change the current process's DEXCR.
+
+Use ``PR_PPC_SET_DEXCR`` with one of ``PR_PPC_DEXCR_CTRL_SET`` or
+``PR_PPC_DEXCR_CTRL_CLEAR`` 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
+
+ * - ``EINVAL``
+ - The ctrl value contains unrecognised flags.
+
+ * - ``EINVAL``
+ - The ctrl value contains mutually conflicting flags (e.g.,
+ ``PR_PPC_DEXCR_CTRL_SET | PR_PPC_DEXCR_CTRL_CLEAR``)
+
+ * - ``EPERM``
+ - This aspect cannot be modified with prctl() (check for the
+ PR_PPC_DEXCR_CTRL_EDITABLE flag with PR_PPC_GET_DEXCR).
+
+ * - ``EPERM``
+ - The process does not have sufficient privilege to perform the operation.
+ For example, clearing NPHIE on exec is a privileged operation (a process
+ can still clear its own NPHIE aspect without privileges).
+
+This interface allows a process to control its own DEXCR aspects, and also set
+the initial DEXCR value for any children in its process tree (up to the next
+child to use an ``*_ONEXEC`` control). This allows fine-grained control over the
+default value of the DEXCR, for example allowing containers to run with different
+default values.
coredump and ptrace
Documents how to use the PR_PPC_GET_DEXCR and PR_PPC_SET_DEXCR prctl()'s for changing a process's DEXCR or its process tree default value. Signed-off-by: Benjamin Gray <bgray@linux.ibm.com> --- Documentation/arch/powerpc/dexcr.rst | 141 ++++++++++++++++++++++++++- 1 file changed, 139 insertions(+), 2 deletions(-)