Allow multifd to use an fdset when migrating to a file. This is useful
for the scenario where the management layer wants to have control over
the migration file.
By receiving the file descriptors directly, QEMU can delegate some
high level operating system operations to the management layer (such
as mandatory access control). The management layer might also want to
add its own headers before the migration stream.
Enable the "file:/dev/fdset/#" syntax for the multifd migration with
fixed-ram. The requirements for the fdset mechanism are:
On the migration source side:
- the fdset must contain two fds that are not duplicates between
themselves;
- if direct-io is to be used, exactly one of the fds must have the
O_DIRECT flag set;
- the file must be opened with WRONLY both times.
On the migration destination side:
- the fdset must contain one fd;
- the file must be opened with RDONLY.
Signed-off-by: Fabiano Rosas <farosas@suse.de>
32f31a3e72