Avi Kivity wrote:
> H. Peter Anvin wrote:
>> 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.)
>>
>
> Strange, the manual lists 0f b0 and 0f b1 as compatible all the way back
> to the 486. What opcode are you seeing?
>
0F A6. Was apparently used on A-step 486's.
Could also just be a nonsense opcode used for trapping, or something
like that.
>> 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?
>
> In kernel. I've never seen a #UD in real mode, that's why it isn't
> handled.
>
Just was a bit surprised to note that it explicitly tests for a small
number of traps, when all the traps can be taken in real mode (via the IVT.)
-hpa
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