[Qemu-devel] Potential sparc32 MMU bug

[Peter <pjcreath+openbios@gmail.com>]

While working on getting SunOS to boot under qemu, I ran into a very
odd bug, and I'm not sure whose fault it is.

The SunOS bootloader tries to install trap 0 by writing to the trap
table.  The trap table is in the .text (read-only) section of the
OpenBIOS ROM.

The bug is that the write to the read-only section silently fails --
it doesn't cause an interrupt in the VM.

It looks like the VM believes all of the ROM is rwx (based on my
examination of cpu_sparc_handle_mmu_fault).  I presume the write fails
because of Linux's memory protection (since the OpenBIOS ELF is
mmap'd).  But I'm not sure why the disallowed write doesn't cause
_something_ to happen.

Should qemu be catching the failed write and passing it on to the VM?
Does qemu need to tell the VM's MMU which portions of the loaded ROM
are read-only?  Or does OpenBIOS need to inform the VM's MMU that the
loaded .text section is read-only?

I presume it's something OpenBIOS should be doing, but that mailing
list is very very quiet, and I figured I'd make sure it wasn't an
underlying qemu bug.

How should qemu be handling this?  What parts of qemu should I look at next?

If people are interested in booting SunOS under qemu, you may want to
check out my posting to the OpenBIOS mailing list, since most (if not
all) of the problems are OpenBIOS's.