migration: Add documentation for fdset with multifd + file

With the last few changes to the fdset infrastructure, we now allow
multifd to use an fdset when migrating to a file. This is useful for
th

migration: Add documentation for fdset with multifd + file

With the last few changes to the fdset infrastructure, we now 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.

Document the "file:/dev/fdset/#" syntax for the multifd migration with
mapped-ram. The requirements for the fdset mechanism are:

- 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 on the migration source side;

- the file must be opened with RDONLY on the migration destination
side.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>

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migration/ram: Introduce 'mapped-ram' migration capability

Add a new migration capability 'mapped-ram'.

The core of the feature is to ensure that RAM pages are mapped
directly to offsets in the res

migration/ram: Introduce 'mapped-ram' migration capability

Add a new migration capability 'mapped-ram'.

The core of the feature is to ensure that RAM pages are mapped
directly to offsets in the resulting migration file instead of being
streamed at arbitrary points.

The reasons why we'd want such behavior are:

- The resulting file will have a bounded size, since pages which are
dirtied multiple times will always go to a fixed location in the
file, rather than constantly being added to a sequential
stream. This eliminates cases where a VM with, say, 1G of RAM can
result in a migration file that's 10s of GBs, provided that the
workload constantly redirties memory.

- It paves the way to implement O_DIRECT-enabled save/restore of the
migration stream as the pages are ensured to be written at aligned
offsets.

- It allows the usage of multifd so we can write RAM pages to the
migration file in parallel.

For now, enabling the capability has no effect. The next couple of
patches implement the core functionality.

Acked-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240229153017.2221-8-farosas@suse.de
Signed-off-by: Peter Xu <peterx@redhat.com>

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