Saturday, 14 November 2009

Booting Ubuntu Karmic with grub2 and EFI

The past few days I've been tinkering with the TianoCore EFI and trying to get Ubuntu Karmic to boot with the EFI enabled grub2. To make debugging easier, I did all this inside a virtualised QEMU environment. A lot of this earlier hackery was blogged about in a previous article.

In trying to get this to work I hit a couple of issues; figuring out workarounds took some thought and time. Some of it is still "work in progress", but at least now I know what needs to be fixed.

Below is a video (grabbed using the recordmydesktop tool) showing Ubuntu Karmic booting:




video

First you see the TianoCore EFI starting and then loading grub2 - this then loads a very slimmed down grub.cfg and eventually boots the system.

I hit several issues; firstly the kernel oops'd when trying to call virt_efi_get_next_variable() - for some reason calling this EFI runtime service support function in the EFI is causing the kernel to oops. Other EFI support functions seem to work correctly, so my current belief is that the bug is not in the kernel EFI driver, but this needs a little more poking around to verify this assumption.

Second issue is that loading some grub2 modules and using some fundamental grub2 commands such as 'set' cause the EFI get caught in a loop. It took me a while to corner this - eventually I cut down the default grub config file down to a really minimal version which could at least boot the kernel! Anyhow, this grub2 issue is first on my list of bugs to fix.

Debugging these issues is a little tricky, but using QEMU helps as I can dump out debug from grub2 over a pseudo serial port which I can capture and view in real-time. Secondly, debugging the kernel using a serial console in QEMU (using the console=ttyS0 boot option) allows me to capture all kernel boot messages (including the offending kernel oops).

I still need to look into ACPI support and how to get the video initialised to get more than bog-standard VGA resolution, but these are currently lower priorities.

Without QEMU and Tristan Gingold's port of the TianoCore EFI to QEMU, this work would have been much harder.

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