[Qemu-devel] [PATCH 0/2] Improve truncation behavior of memory-backend-file

[Haozhong Zhang <haozhong.zhang@intel.com>]

The previous discussion thread can be found at
http://lists.nongnu.org/archive/html/qemu-devel/2016-10/msg04668.html

For a mmeory backend file, e.g.
    -object memory-backend-file,mem-path=foo,size=SZ,...
the size of the backend file 'foo' is specified by the 'size'
option. If the specified size 'SZ' does not match the actual size of
file 'foo', QEMU will truncate the backend file 'foo'. In certain
usage scenarios (e.g. vNVDIMM), the truncation may corrupt the
existing data in the file.

Patch 1 in this series avoids such data corruption by disabling
truncating non-empty backend files. If the backend file is not empty
and the option 'size' does not match the actual file size, QEMU will
error out. If the backend file is empty, QEMU will extend to the size
specified by the option 'size'.

Patch 2 makes the option 'size' optional. It's to avoid the misusing
of 'size' option. If the user is uncertain about the backend file size,
they can skip the 'size' option and let QEMU use the actual file size.
For an empty file, the option 'size' must be specified.

Haozhong Zhang (2):
  exec.c: do not truncate non-empty memory backend file
  hostmem-file: allow option 'size' optional

 backends/hostmem-file.c | 10 ++++++----
 exec.c                  | 45 ++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 50 insertions(+), 5 deletions(-)

-- 
2.10.1