kvm: repeatable kernel crash with Athlon II cpu

[Stas Sergeev]

Hello.

I've been running dosemu and found out that it hangs
or reboots one of my PCs. This happens with any fedora-23
kernels and hand-compiled kernels. The latest I tried, were:
4.4.6-300.fc23.x86_64
4.5.0-rc6

I tried to put the debug printfs into dosemu, and it seems
the crash happens on KVM_RUN ioctl, but of course the one
can't say for sure when an instant reboot happens. Yet if
I disable the KVM use, or try on a different CPU, there is
no crash too, so I am pretty sure it is the KVM bug.

The CPU on which the problem happens:
AMD Athlon(tm) II X2 245 Processor

The CPUs on which the problem doesn't happen:
AMD FX 8350
Intel(R) Core(TM) i7-5500U CPU @ 2.40GHz

I've set up the test-case here:
https://github.com/stsp/dosemu2/tree/bug

After checking out this branch, you need to get the file
c_drive/c.tar.gz
and unpack it into ~/.dosemu/drives so that ~/.dosemu/drives/c
exists.
After that you need to build and run dosemu.
Make sure to install slang-devel first, so that it can work
on a text-console (kms or non-kms).
Then run dosemu. Dos Navigator should load. Hit Alt-x to exit,
then Enter to confirm. At this point the host machine reboots,
or, if there is a different CPU - dosemu just exits.
The test must be repeated 2-3 times to make sure, but in most
cases it works from the first time.

Hope someone can find such an old CPU an reproduce the problem.
Please let me know what info should I provide or what debugging
to do to help getting that fixed.