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[Libguestfs] Attempts to install a Windows driver from WinPE



One of the things we would really like to be able to do for V2V is to install a new driver in a Windows guest. There are a couple of reasons for this:

* The guest may not be bootable without the driver installed, for example because the underlying virtual hardware has changed from vmscsi to virtio.

* If the guest can boot, the alternative is to modify the guest to run a script on next boot. This requires making assumptions about supporting software being installed and working correctly on the guest. Certain environments, particularly heavily locked-down environments, make this an unsafe assumption.

The Windows PE environment looks perfect for this task. It gives you a very lightweight Windows OS which can be customised with additional tools. It is specifically for doing installations. I spent Friday trying to use it to install a driver in a guest. Here's what I tried and why it didn't work.

Installing a driver in Windows is 'driven' by a .inf file. From my (admittedly limited) understanding, this broadly describes:

* The files which need to be installed
* The hardware the driver is compatible with

The files, including the .inf file itself must be copied in to the correct places. In addition, information from the .inf file must be written to the registry. It is this last part which causes problems.

From reading documentation, it appears that a driver would normally be installed using the SetupCopyOEMInf() library call. I wrote a simple wrapper round this and installed it in the Windows PE image, along with the VirtIO drivers. I booted into Windows PE and attempted to install the driver. As you might expect, the drivers were installed into the Windows PE image rather than the guest. I then tried setting %systemdrive% and %systemroot% to the guest image. This appears to have no effect. This is what makes me suspect that the process is primarily registry driven.

I started looking around for ways of using a different registry. I discovered the Registry Editor PE plugin to BartPE (https://sourceforge.net/projects/regeditpe/) which allows editing the registry of a guest. Looking at how it does this, it uses reg.exe to load the guests's hives. I confirmed that you can do this. Unfortunately you don't seem to be able to replace the default hives. The new hives are loaded in a different part of the tree, and are therefore ignored.

This is as far as I've got. Still on my list are:

* Ask on various Windows mailing lists how to do this
* Investigate if the packaged .msi containing the drivers is more flexible.
* Look for other, possibly lower level, ways of replacing making a process use a different registry.

Any and all suggestions are gratefully received,

Matt
--
Matthew Booth, RHCA, RHCSS
Red Hat Engineering, Virtualisation Team

M:       +44 (0)7977 267231
GPG ID:  D33C3490
GPG FPR: 3733 612D 2D05 5458 8A8A 1600 3441 EA19 D33C 3490


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