[PATCH v4 12/15] xen/arm: Document the errata implemented in Xen

[Julien Grall <julien.grall@arm.com>]

The new document will help to keep track of each erratum Xen is able to
handle.

The text is based on the Linux doc in Documents/arm64/silicon-errata.txt.

Also list the current errata that Xen is aware of.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>

---
    Changes in v4:
        - Fix grammar in the document : s/by ARM64/on ARM64/
        - Add Stefano's acked-by

    Changes in v3:
        - Fix grammar in the commit message
        - s/Linux/Xen/
        - Mention that runtime patching is only supported by ARM64
---
 docs/misc/arm/silicon-errata.txt | 45 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 45 insertions(+)
 create mode 100644 docs/misc/arm/silicon-errata.txt

diff --git a/docs/misc/arm/silicon-errata.txt b/docs/misc/arm/silicon-errata.txt
new file mode 100644
index 0000000..5d54694
--- /dev/null
+++ b/docs/misc/arm/silicon-errata.txt
@@ -0,0 +1,45 @@
+                Silicon Errata and Software Workarounds
+                =======================================
+
+It is an unfortunate fact of life that hardware is often produced with
+so-called "errata", which can cause it to deviate from the architecture
+under specific circumstances.  For hardware produced by ARM, these
+errata are broadly classified into the following categories:
+
+  Category A: A critical error without a viable workaround.
+  Category B: A significant or critical error with an acceptable
+              workaround.
+  Category C: A minor error that is not expected to occur under normal
+              operation.
+
+For more information, consult one of the "Software Developers Errata
+Notice" documents available on infocenter.arm.com (registration
+required).
+
+As far as Xen is concerned, Category B errata may require some special
+treatment in the hypervisor. For example, avoiding a particular sequence
+of code, or configuring the processor in a particular way. A less common
+situation may require similar actions in order to declassify a Category A
+erratum into a Category C erratum. These are collectively known as
+"software workarounds" and are only required in the minority of cases
+(e.g. those cases that both require a non-secure workaround *and* can
+be triggered by Xen).
+
+For software workarounds that may adversely impact systems unaffected by
+the erratum in question, a Kconfig entry is added under "ARM errata
+workarounds via the alternatives framework". These are enabled by default
+and patched in at runtime when an affected CPU is detected. Note that
+runtime patching is only supported on ARM64. For less-intrusive workarounds,
+a Kconfig option is not available and the code is structured (preferably
+with a comment) in such a way that the erratum will not be hit.
+
+This approach can make it slightly onerous to determine exactly which
+errata are worked around in an arbitrary hypervisor source tree, so this
+file acts as a registry of software workarounds in the Xen hypervisor and
+will be updated when new workarounds are committed and backported to
+stable hypervisors.
+
+| Implementor    | Component       | Erratum ID      | Kconfig                 |
++----------------+-----------------+-----------------+-------------------------+
+| ARM            | Cortex-A15      | #766422         | N/A                     |
+| ARM            | Cortex-A57      | #852523         | N/A                     |
-- 
1.9.1


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