Xen Copy-On-Write Checkpoints

Overview

This modified version of Xen 3.2.2 provides the ability to take copy-on-write checkpoints of running VMs (PV and HVM). Its design and implementation is described in the 2010 report, "Fast, Lightweight Virtual Machine Checkpointing".

Assumptions

• 32-bit Xen hypervisor & 32-bit PV or 32-bit HVM guest
• PAE mode not supported
• SHADOW OPTIMIZATIONS turned off and SHOPT_PREFETCH turned off

Implementation Details

• Shadow log-dirty mode is utilized to intercept memory writes. All pages are marked as read-only. Writes are intercepted, target page is marked dirty, and page made writable.

• CoW hypercall allows CoW checkpoint mode to be turned on/off. Turns shadow log-dirty mode on/off and marks hypervisor in CoW mode.

• Properly handles competing writes from dom0 copy process and CoW handler. CoW copies occur to separate 'diff' buffer which are then merged on top of actual copy buffer used by dom0 copy process.

• The xm save command has been overridden to always perform CoW checkpoints, i.e. the -c option has no use/meaning.

Author

Mike Sun <[email protected]>

Acknowledgements

Thanks to Artem Dinaburg for providing code to walk shadow page tables, Mike Hibler and the Emulab for their help in setting up a testbed, and Doug Blough for his guidance.