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Documentation: Document PowerPC kernel dynamic DEXCR interface
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> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au> Link: https://msgid.link/20240417112325.728010-10-bgray@linux.ibm.com
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Benjamin Gray
Michael Ellerman
parent
f88723a609
commit
9248edf31a
@ -36,8 +36,145 @@ state for a process.
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Configuration
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=============
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The DEXCR is currently unconfigurable. All threads are run with the
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NPHIE aspect enabled.
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prctl
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-----
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A process can control its own userspace DEXCR value using the
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``PR_PPC_GET_DEXCR`` and ``PR_PPC_SET_DEXCR`` pair of
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:manpage:`prctl(2)` commands. These calls have the form::
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prctl(PR_PPC_GET_DEXCR, unsigned long which, 0, 0, 0);
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prctl(PR_PPC_SET_DEXCR, unsigned long which, unsigned long ctrl, 0, 0);
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The possible 'which' and 'ctrl' values are as follows. Note there is no relation
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between the 'which' value and the DEXCR aspect's index.
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.. flat-table::
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:header-rows: 1
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:widths: 2 7 1
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* - ``prctl()`` which
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- Aspect name
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- Aspect index
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* - ``PR_PPC_DEXCR_SBHE``
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- Speculative Branch Hint Enable (SBHE)
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- 0
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* - ``PR_PPC_DEXCR_IBRTPD``
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- Indirect Branch Recurrent Target Prediction Disable (IBRTPD)
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- 3
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* - ``PR_PPC_DEXCR_SRAPD``
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- Subroutine Return Address Prediction Disable (SRAPD)
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- 4
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* - ``PR_PPC_DEXCR_NPHIE``
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- Non-Privileged Hash Instruction Enable (NPHIE)
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- 5
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.. flat-table::
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:header-rows: 1
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:widths: 2 8
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* - ``prctl()`` ctrl
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- Meaning
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* - ``PR_PPC_DEXCR_CTRL_EDITABLE``
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- This aspect can be configured with PR_PPC_SET_DEXCR (get only)
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* - ``PR_PPC_DEXCR_CTRL_SET``
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- This aspect is set / set this aspect
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* - ``PR_PPC_DEXCR_CTRL_CLEAR``
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- This aspect is clear / clear this aspect
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* - ``PR_PPC_DEXCR_CTRL_SET_ONEXEC``
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- This aspect will be set after exec / set this aspect after exec
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* - ``PR_PPC_DEXCR_CTRL_CLEAR_ONEXEC``
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- This aspect will be clear after exec / clear this aspect after exec
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Note that
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* which is a plain value, not a bitmask. Aspects must be worked with individually.
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* ctrl is a bitmask. ``PR_PPC_GET_DEXCR`` returns both the current and onexec
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configuration. For example, ``PR_PPC_GET_DEXCR`` may return
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``PR_PPC_DEXCR_CTRL_EDITABLE | PR_PPC_DEXCR_CTRL_SET |
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PR_PPC_DEXCR_CTRL_CLEAR_ONEXEC``. This would indicate the aspect is currently
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set, it will be cleared when you run exec, and you can change this with the
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``PR_PPC_SET_DEXCR`` prctl.
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* The set/clear terminology refers to setting/clearing the bit in the DEXCR.
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For example::
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prctl(PR_PPC_SET_DEXCR, PR_PPC_DEXCR_IBRTPD, PR_PPC_DEXCR_CTRL_SET, 0, 0);
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will set the IBRTPD aspect bit in the DEXCR, causing indirect branch prediction
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to be disabled.
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* The status returned by ``PR_PPC_GET_DEXCR`` represents what value the process
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would like applied. It does not include any alternative overrides, such as if
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the hypervisor is enforcing the aspect be set. To see the true DEXCR state
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software should read the appropriate SPRs directly.
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* The aspect state when starting a process is copied from the parent's state on
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:manpage:`fork(2)`. The state is reset to a fixed value on
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:manpage:`execve(2)`. The PR_PPC_SET_DEXCR prctl() can control both of these
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values.
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* The ``*_ONEXEC`` controls do not change the current process's DEXCR.
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Use ``PR_PPC_SET_DEXCR`` with one of ``PR_PPC_DEXCR_CTRL_SET`` or
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``PR_PPC_DEXCR_CTRL_CLEAR`` to edit a given aspect.
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Common error codes for both getting and setting the DEXCR are as follows:
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.. flat-table::
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:header-rows: 1
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:widths: 2 8
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* - Error
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- Meaning
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* - ``EINVAL``
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- The DEXCR is not supported by the kernel.
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* - ``ENODEV``
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- The aspect is not recognised by the kernel or not supported by the
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hardware.
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``PR_PPC_SET_DEXCR`` may also report the following error codes:
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.. flat-table::
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:header-rows: 1
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:widths: 2 8
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* - Error
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- Meaning
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* - ``EINVAL``
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- The ctrl value contains unrecognised flags.
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* - ``EINVAL``
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- The ctrl value contains mutually conflicting flags (e.g.,
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``PR_PPC_DEXCR_CTRL_SET | PR_PPC_DEXCR_CTRL_CLEAR``)
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* - ``EPERM``
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- This aspect cannot be modified with prctl() (check for the
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PR_PPC_DEXCR_CTRL_EDITABLE flag with PR_PPC_GET_DEXCR).
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* - ``EPERM``
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- The process does not have sufficient privilege to perform the operation.
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For example, clearing NPHIE on exec is a privileged operation (a process
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can still clear its own NPHIE aspect without privileges).
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This interface allows a process to control its own DEXCR aspects, and also set
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the initial DEXCR value for any children in its process tree (up to the next
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child to use an ``*_ONEXEC`` control). This allows fine-grained control over the
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default value of the DEXCR, for example allowing containers to run with different
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default values.
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coredump and ptrace
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