qubes-doc/en/developers/services/dvm-impl.md

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DisposableVM implementation in Qubes

DisposableVM image preparation

DisposableVM is not started like other VMs, by executing equivalent of xl create - it would be too slow. Instead, DisposableVM are started by restore from a savefile.

Preparing a savefile is done by /usr/lib/qubes/qubes_prepare_saved_domain.sh script. It takes two mandatory arguments, appvm name (APPVM) and the savefile name, and optional path to "prerun" script. The script executes the following steps:

1. APPVM is started by qvm-start

2. xenstore key /local/domain/appvm_domain_id/qubes_save_request is created

3. if prerun script was specified, copy it to qubes_save_script xenstore key

4. wait for the qubes_used_mem key to appear

5. (in APPVM) APPVM boots normally, up to the point in /etc/init.d/qubes_core script when the presence of qubes_save_request key is tested. If it exists, then

   1. (in APPVM) if exists, prerun script is retrieved from the respective xenstore key and executed. This preloads filesystem cache with useful applications, so that they will start faster.
   2. (in APPVM) the amount of used memory is stored to qubes_used_mem xenstore key
   3. (in APPVM) busy-waiting for qubes_restore_complete xenstore key to appear

6. when qubes_used_mem key appears, the domain memory is reduced to this amount, to make the savefile smaller.

7. APPVM private image is detached

8. the domain is saved via xl save

9. the COW file volatile.img (cow for for root fs and swap) is packed to saved_cows.tar archive

qubes_prepare_saved_domain.sh script is somehow lowlevel. It is usually called by qvm-create-default-dvm script, that takes care of creating a special AppVM (named template_name-dvm) to be passed to qubes_prepare_saved_domain.sh, as well as copying the savefile to /dev/shm (the latter action is not done if the /var/lib/qubes/dvmdata/dont_use_shm file exists).

Restoring a DisposableVM from the savefile

Normally, disposable VM is created when qubes rpc request with target $dispvm is received. Then, as a part of rpc connection setup, the qfile-daemon-dvm program is executed; it executes /usr/lib/qubes/qubes_restore program. It is crucial that this program executes quickly, to make DisposableVM creation overhead bearable for the user. Its main steps are:

1. modify the savefile so that the VM name, VM UUID, MAC address and IP address are unique
2. restore the COW files from the saved_cows.tar
3. create the /var/run/qubes/fast_block_attach file, whose presence tells the /etc/xen/scripts/block script to bypass some redundant checks and execute as fast as possible.
4. execute "xl restore" in order to restore a domain.
5. create the same xenstore keys as normally created when AppVM boots (e.g. qubes_ip)
6. create the qubes_restore_complete xenstore key. This allows the boot process in DisposableVM to continue.

The actual passing of files between AppVM and a DisposableVM is implemented via qubes rpc.

Validating the DisposableVM savefile

DisposableVM savefile contains references to template rootfs and to COW files. The COW files are restored before each DisposableVM start, so they cannot change. On the other hand, if templateVM is started, the template rootfs will change, and it may not be coherent with the COW files.

Therefore, the check for template rootfs modification time being older than DisposableVM savefile modification time is required. It is done in qfilexchgd daemon, just before restoring DisposableVM. If necassary, an attempt is made to recreate the DisposableVM savefile, using the last template used (or default template, if run for the first time) and the default prerun script, residing at /var/lib/qubes/vm-templates/templatename/dispvm_prerun.sh. Unfortunately, the prerun script takes a lot of time to execute - therefore, after template rootfs modification, the next DisposableVM creation can be longer by about 2.5 minutes.