[PATCH v2 09/12] Documentation: Document PowerPC kernel DEXCR interface

[Benjamin Gray]

Describe the DEXCR and document how to configure it.

Signed-off-by: Benjamin Gray <bgray at linux.ibm.com>

---
v2:	* Document coredump & ptrace support
v1:	* Remove the dynamic control docs, describe the static config
	  option

This documentation is a little bare for now, but will be expanded on
when dynamic DEXCR control is added.
---
 Documentation/powerpc/dexcr.rst | 58 +++++++++++++++++++++++++++++++++
 Documentation/powerpc/index.rst |  1 +
 2 files changed, 59 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..75f1efaa8dc5
--- /dev/null
+++ b/Documentation/powerpc/dexcr.rst
@@ -0,0 +1,58 @@
+.. SPDX-License-Identifier: GPL-2.0-or-later
+
+==========================================
+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 privileged 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.
+
+
+Kernel Config
+=============
+
+The kernel supports a static default DEXCR value determined at config time.
+Set the ``PPC_DEXCR_DEFAULT`` config to the value you want all processes to
+use.
+
+
+coredump and ptrace
+===================
+
+The userspace values of the DEXCR and HDEXCR (in this order) are exposed under
+``NT_PPC_DEXCR``. These are each 32 bits and readonly, and are intended to
+assist with core dumps. The DEXCR may be made writable in future.
+
+If the kernel config ``CONFIG_CHECKPOINT_RESTORE`` is enabled, then
+``NT_PPC_HASHKEYR`` is available and exposes the HASHKEYR value of the process
+for reading and writing. This is a tradeoff between increased security and
+checkpoint/restore support: a process should normally have no need to know its
+secret key, but restoring a process requires setting its original key. The key
+therefore appears in core dumps, and an attacker may be able to retrieve it from
+a coredump and effectively bypass ROP protection on any threads that share this
+key (potentially all threads from the same parent that have not run ``exec()``).
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
-- 
2.39.2