Add memfd based hostmem

Add a new memory backend, similar to hostmem-file, except that it
doesn't need to create files. It also enforces memory sealing.

This backend is mainly useful for sharing the memory with other
processes.

Note that Linux supports transparent huge-pages of shmem/memfd memory
since 4.8. It is relatively easier to set up THP than a dedicate
hugepage mount point by using "madvise" in
/sys/kernel/mm/transparent_hugepage/shmem_enabled.

Since 4.14, memfd allows to set hugetlb requirement explicitly.

Pending for merge in 4.16 is memfd sealing support for hugetlb backed
memory.

Usage:
-object memory-backend-memfd,id=mem1,size=1G

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20180201132757.23063-5-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 22 insertions, 0 deletions

diff --git a/qemu-options.hx b/qemu-options.hx
index 8ce427d..265cf9f 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -4024,6 +4024,28 @@ Memory backend objects offer more control than the @option{-m} option that is
 traditionally used to define guest RAM. Please refer to
 @option{memory-backend-file} for a description of the options.
 
+@item -object memory-backend-memfd,id=@var{id},merge=@var{on|off},dump=@var{on|off},prealloc=@var{on|off},size=@var{size},host-nodes=@var{host-nodes},policy=@var{default|preferred|bind|interleave},seal=@var{on|off},hugetlb=@var{on|off},hugetlbsize=@var{size}
+
+Creates an anonymous memory file backend object, which allows QEMU to
+share the memory with an external process (e.g. when using
+vhost-user). The memory is allocated with memfd and optional
+sealing. (Linux only)
+
+The @option{seal} option creates a sealed-file, that will block
+further resizing the memory ('on' by default).
+
+The @option{hugetlb} option specify the file to be created resides in
+the hugetlbfs filesystem (since Linux 4.14).  Used in conjunction with
+the @option{hugetlb} option, the @option{hugetlbsize} option specify
+the hugetlb page size on systems that support multiple hugetlb page
+sizes (it must be a power of 2 value supported by the system).
+
+In some versions of Linux, the @option{hugetlb} option is incompatible
+with the @option{seal} option (requires at least Linux 4.16).
+
+Please refer to @option{memory-backend-file} for a description of the
+other options.
+
 @item -object rng-random,id=@var{id},filename=@var{/dev/random}
 
 Creates a random number generator backend which obtains entropy from