postcopy: Add doc about hugepages and postcopy

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <20170224182844.32452-16-dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/docs/migration.txt b/docs/migration.txt
index c8b0304..1b940a8 100644
--- a/docs/migration.txt
+++ b/docs/migration.txt
@@ -540,3 +540,16 @@ request for a page that has already been sent is ignored.  Duplicate requests
 such as this can happen as a page is sent at about the same time the
 destination accesses it.
 
+=== Postcopy with hugepages ===
+
+Postcopy now works with hugetlbfs backed memory:
+  a) The linux kernel on the destination must support userfault on hugepages.
+  b) The huge-page configuration on the source and destination VMs must be
+     identical; i.e. RAMBlocks on both sides must use the same page size.
+  c) Note that -mem-path /dev/hugepages  will fall back to allocating normal
+     RAM if it doesn't have enough hugepages, triggering (b) to fail.
+     Using -mem-prealloc enforces the allocation using hugepages.
+  d) Care should be taken with the size of hugepage used; postcopy with 2MB
+     hugepages works well, however 1GB hugepages are likely to be problematic
+     since it takes ~1 second to transfer a 1GB hugepage across a 10Gbps link,
+     and until the full page is transferred the destination thread is blocked.