qemu: Add support for mapped-ram on save

qemu: Add support for mapped-ram on save

Introduce support for QEMU's new mapped-ram stream format [1].
mapped-ram can be enabled by setting the 'save_image_format'
setting in qemu.conf to 'sparse'.

To use mapped-ram with QEMU:
- The 'mapped-ram' migration capability must be set to true
- The 'multifd' migration capability must be set to true and
  the 'multifd-channels' migration parameter must set to 1
- QEMU must be provided an fdset containing the migration fd
- The 'migrate' qmp command is invoked with a URI referencing the
  fdset and an offset where to start reading or writing the data
  stream, e.g.

  {"execute":"migrate",
   "arguments":{"detach":true,"resume":false,
                "uri":"file:/dev/fdset/0,offset=0x11921"}}

The mapped-ram stream, in conjunction with direct IO and multifd
support provided by subsequent patches, can significantly improve
the time required to save VM memory state. The following tables
compare mapped-ram with the existing, sequential save stream. In
all cases, the save and restore operations are to/from a block
device comprised of two NVMe disks in RAID0 configuration with
xfs (~8600MiB/s). The values in the 'save time' and 'restore time'
columns were scraped from the 'real' time reported by time(1). The
'Size' and 'Blocks' columns were provided by the corresponding
outputs of stat(1).

VM: 32G RAM, 1 vcpu, idle (shortly after boot)

                       | save    | restore |
		       | time    | time    | Size         | Blocks
-----------------------+---------+---------+--------------+--------
legacy                 | 6.193s  | 4.399s  | 985744812    | 1925288
-----------------------+---------+---------+--------------+--------
mapped-ram             | 5.109s  | 1.176s  | 34368554354  | 1774472
-----------------------+---------+---------+--------------+--------
legacy + direct IO     | 5.725s  | 4.512s  | 985765251    | 1925328
-----------------------+---------+---------+--------------+--------
mapped-ram + direct IO | 4.627s  | 1.490s  | 34368554354  | 1774304
-----------------------+---------+---------+--------------+--------
mapped-ram + direct IO |         |         |              |
 + multifd-channels=8  | 4.421s  | 0.845s  | 34368554318  | 1774312
-------------------------------------------------------------------

VM: 32G RAM, 30G dirty, 1 vcpu in tight loop dirtying memory

                       | save    | restore |
		       | time    | time    | Size         | Blocks
-----------------------+---------+---------+--------------+---------
legacy                 | 25.800s | 14.332s | 33154309983  | 64754512
-----------------------+---------+---------+--------------+---------
mapped-ram             | 18.742s | 15.027s | 34368559228  | 64617160
-----------------------+---------+---------+--------------+---------
legacy + direct IO     | 13.115s | 18.050s | 33154310496  | 64754520
-----------------------+---------+---------+--------------+---------
mapped-ram + direct IO | 13.623s | 15.959s | 34368557392  | 64662040
-----------------------+-------- +---------+--------------+---------
mapped-ram + direct IO |         |         |              |
 + multifd-channels=8  | 6.994s  | 6.470s  | 34368554980  | 64665776
--------------------------------------------------------------------

As can be seen from the tables, one caveat of mapped-ram is the logical
file size of a saved image is basically equivalent to the VM memory size.
Note however that mapped-ram typically uses fewer blocks on disk, hence
the name 'sparse' for 'save_image_format'.

Also note the mapped-ram stream is incompatible with the existing stream
format, hence mapped-ram cannot be used to restore an image saved with
the existing format and vice versa.

[1] https://gitlab.com/qemu-project/qemu/-/blob/master/docs/devel/migration/mapped-ram.rst?ref_type=heads

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>