Friday, 6 November 2009

QEMU and EFI BIOS hackery

Earlier this week I blogged about QEMU and EFI BIOS. Trying to debug a problem with grub2-efi-ia32 has given me a few little headaches but I'm finding ways to work around them all.

The first issue is getting a system installed with an EFI BIOS. My quick hack was to create a 4GB QEMU qcow2 disk image and then inside this create a small EFI FAT12 boot partition using fdisk - (partition type 0xef in the 1st primary partition). I then installed Ubuntu Karmic Desktop with ext4 and swap in primary partitions 2 and 3 by booting with the conventional BIOS. I then installed grub2-efi-ia32 in the EFI boot partition and then booted QEMU using the TianoCore EFI BIOS that has been ported to QEMU.

One problem is that the EFI BIOS does not scroll the screen, hence all output when it reaches the end of the screen just keeps over writing the last line, making debugging with printf() style prints nearly impossible. Then I found that the BIOS emits characters over a serial port, which QEMU can emulate. Unfortunately, the output contains VT control characters to do cursor positioning and pretty console colours, which makes reading the output a little painful. So I hacked up a simple tool to take the output from QEMU and strip out the VT control chars to make the text easier to read.

Now QEMU boot line is as follows:

qemu -bios bios-efi.bin -m 1024 karmic-efi-qcow2.img -serial stdio 2>&1 | ./parse-output

..and this dumps the output from the BIOS and grub2 to stdout in a more readable form.

The parse-output tool is a little hacky - but does the job. For reference, I've put it in my debug repository here.

Grub2 allows one to enable some level of debugging output by issuing the following command:

set debug=all

..which gives me some idea of what's working or broken at a fairly low enough level before I start attaching gdb. Fortunately debugging using gdb has been fairly well documented here - I just now need to shoe-horn in small patch to allow me to attach gdb to grub2 from outside QEMU - but that's for another debug session...

1 comment:

  1. Ummmm....wouldn't you want to create a GPT disklabel with parted instead of an MSDOS disklabel with fdisk? I thought The Right Thing(tm) to do for (U)EFI was to use GPT. Just sayin'... I'm just starting to hack around with UEFI and GPT. I already have most of my systems as GPT (because Linux supports it).