Just for fun, I tried to boot OS/2 Warp 4.0 under KVM (KVM-59 with the
latest git kernel from Linus as of yesterday, slightly post 2.6.24-rc8.)
I found that it crashes very early, apparently because KVM doesn't
handle an #UD received in user mode. It appears that OS/2 actually
provokes an #UD deliberately in real mode, from the disassembly it looks
like it's trying to probe for the 486 version of cmpxchg (which has a
different opcode than the 586+ version.)
It looks like the kernel code filters out a very small number of
real-mode exceptions, and does a KVM exit for all the other ones; the
userspace code then unconditionally barfs. This is presumably a
temporary hack; what is the intended behaviour - for this to be handled
in-kernel, or in userspace?
-hpa
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