Mathias Gottschlag wrote:
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.
I looked briefly and there didn't appear to be any explicit checks for
wrap around but I think that it will work correctly since we're always
using at least unsigned long in the host. My suspicion is that we're
somehow getting something wrong with MMIO decoding.
Regards,
Anthony Liguori
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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