I am currently working on my own hobby kernel and tested it on kvm
today. I think I found a bug in kvm with it (already discussed it on IRC):
I wanted my kernel to be at a higher virtual address, in my case
0xE0000000. To get it there (Grub loaded it at physical addr 0x100000),
I followed the steps at http://wiki.osdev.org/Higher_Half_With_GDT:
Grub inits the CPU with no paging and flat segmenting, I now activate
segments to move the code to 0xE0000000 and later activate paging and
reset the segments. When I run this code on qemu or on real hardware
(Athlon XP), everything works well, but on kvm I get several hangs. For
example I try to write to 0xE00B8000 to write into the VGA framebuffer.
This addres should now get translated back to 0xB8000 by the segment
which has the base address 0x20000000, and this definately works on real
hardware, but on kvm I only notice a hang at the instruction which
writes at that address (kvm still responds, but doesn't update eip
anymore, execution stops.
Someone on IRC told me that this might be because address wrapping isn't
implemented properly (this could have been unnoticed until now as no
real OS uses such weird segmenting).
You can try it out yourself using the code at
http://wiki.osdev.org/Higher_Half_With_GDT, this code will work for
example in qemu, but not in kvm. I attached a version including a build
script and a script to create the floppy image (you only need gcc, nasm
and grub, type in "sh build.sh" and you get your image) for testing. It
should show "Hello world" on a machine where it runs correctly.
Mathias Gottschlag
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