[PATCH v2 6/6] mm: userfaultfd: add basic documentation for UFFDIO_POISON

[Axel Rasmussen]

Just describe the feature at a really basic level.

Signed-off-by: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
---
 Documentation/admin-guide/mm/userfaultfd.rst | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/Documentation/admin-guide/mm/userfaultfd.rst b/Documentation/admin-guide/mm/userfaultfd.rst
index 7c304e432205..b19053436369 100644
--- a/Documentation/admin-guide/mm/userfaultfd.rst
+++ b/Documentation/admin-guide/mm/userfaultfd.rst
@@ -244,6 +244,21 @@ write-protected (so future writes will also result in a WP fault). These ioctls
 support a mode flag (``UFFDIO_COPY_MODE_WP`` or ``UFFDIO_CONTINUE_MODE_WP``
 respectively) to configure the mapping this way.
 
+Memory Poisioning Emulation
+---------------------------
+
+In response to a fault (either missing or minor), an action userspace can
+take to "resolve" it is to issue a ``UFFDIO_POISON``. This will cause any
+future faulters to either get a SIGBUS, or in KVM's case the guest will
+receive an MCE as if there were hardware memory poisoning.
+
+This is used to emulate hardware memory poisoning. Imagine a VM running on a
+machine which experiences a real hardware memory error. Later, we live migrate
+the VM to another physical machine. Since we want the migration to be
+transparent to the guest, we want that same address range to act as if it was
+still poisoned, even though it's on a new physical host which ostentisbly
+doesn't have a memory error in the exact same spot.
+
 QEMU/KVM
 ========
 
-- 
2.41.0.255.g8b1d071c50-goog