mm: userfaultfd: document and enable new UFFDIO_POISON feature

Update the userfaultfd API to advertise this feature as part of feature
flags and supported ioctls (returned upon registration).

Add basic documentation describing the new feature.

Link: https://lkml.kernel.org/r/20230707215540.2324998-7-axelrasmussen@google.comSigned-off-by: Axel Rasmussen <axelrasmussen@google.com>
Acked-by: Peter Xu <peterx@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Brian Geffon <bgeffon@google.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Gaosheng Cui <cuigaosheng1@huawei.com>
Cc: Huang, Ying <ying.huang@intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: James Houghton <jthoughton@google.com>
Cc: Jan Alexander Steffens (heftig) <heftig@archlinux.org>
Cc: Jiaqi Yan <jiaqiyan@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Nadav Amit <namit@vmware.com>
Cc: Naoya Horiguchi <naoya.horiguchi@nec.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suleiman Souhlal <suleiman@google.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: T.J. Alumbaugh <talumbau@google.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: ZhangPeng <zhangpeng362@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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 ostensibly
+doesn't have a memory error in the exact same spot.
+
 QEMU/KVM
 ========
 

include/uapi/linux/userfaultfd.h

@@ -39,7 +39,8 @@
 			   UFFD_FEATURE_MINOR_SHMEM |		\
 			   UFFD_FEATURE_EXACT_ADDRESS |		\
 			   UFFD_FEATURE_WP_HUGETLBFS_SHMEM |	\
-			   UFFD_FEATURE_WP_UNPOPULATED)
+			   UFFD_FEATURE_WP_UNPOPULATED |	\
+			   UFFD_FEATURE_POISON)
 #define UFFD_API_IOCTLS				\
 	((__u64)1 << _UFFDIO_REGISTER |		\
 	 (__u64)1 << _UFFDIO_UNREGISTER |	\
@@ -49,12 +50,14 @@
 	 (__u64)1 << _UFFDIO_COPY |		\
 	 (__u64)1 << _UFFDIO_ZEROPAGE |		\
 	 (__u64)1 << _UFFDIO_WRITEPROTECT |	\
-	 (__u64)1 << _UFFDIO_CONTINUE)
+	 (__u64)1 << _UFFDIO_CONTINUE |		\
+	 (__u64)1 << _UFFDIO_POISON)
 #define UFFD_API_RANGE_IOCTLS_BASIC		\
 	((__u64)1 << _UFFDIO_WAKE |		\
 	 (__u64)1 << _UFFDIO_COPY |		\
+	 (__u64)1 << _UFFDIO_WRITEPROTECT |	\
 	 (__u64)1 << _UFFDIO_CONTINUE |		\
-	 (__u64)1 << _UFFDIO_WRITEPROTECT)
+	 (__u64)1 << _UFFDIO_POISON)
 
 /*
  * Valid ioctl command number range with this API is from 0x00 to