Re: KVM and OS/2: #UD in real mode

["H. Peter Anvin" <hpa-YMNOUZJC4hwAvxtiuMwx3w@public.gmane.org>]

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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