[Hackathon] Restartable driver domains and restartable dom0 notes

[David Vrabel <dvrabel@cantab.net>]

Restartable driver domains and dom0
-----------------------------------

Jeurgen:  See XenSummit presentation.

- Xenstore Domain (bits missing in libxl)
- Looking at driver domains next.

One domain per backend or several backends in one domain.
Drivers crash -> restart driver domains while guests running.
Issues:

- Guest configuration has both backend (e.g., which disk) and frontend
  parameters (e.g., which domain has the backend).
- Frontend needs to reconnect to new backend in new backend.

Suggestions:

- Hack: Split configuration -> backend configuration to backend domain
  configuration.

- Juergen suggests using XenStore (or other extermal database) to
  convey configuration information -- libxl writes this based on
  frontend guest configuration.

- Must be in XenStore (or external) because toolstack domain may be
  restarted.  But in "/toolstack/" top-level node.

1.  Backend driver started, which backends to start (if needed), any
  devices assigned.

2. libxl reads guest config, writes to persistent store configuration
  of backend.

3. libxl reads persistent store, writes backend nodes to current
  driver domain instance (does this on driver domain restart).

* Restart dom0.

Move drivers into dom0 for resource usage => need to restart dom0.

- performance problems with lots of vcpus.

- SATA needs to be in dom0 -- shared by dom0 and guests (similar for
  other devices).

- Can restart any backends in dom0.

Juergens suggestions:

1. dom0 when it starts, "kexec" load's another image (kernel, initrd,
   DT, etc.).

2. hypervisor jumps to loaded image when dom0 starts, providing same
   resources (device, interrupts) that the old domain had.

3. New dom0 restarts any backends.

4. Reads persistent store, starts any backend instances as needed.

Security concerns on loaded image.  Is it trusted?

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