Re: [Regression] kvm-userspace: VM freezes after booting FreeDOS

[Avi Kivity <avi@qumranet.com>]

Jan Kiszka wrote:
>>>     
>>>       
>> But it seems the best thing to do is not to trap #DB in
>> update_exception_bitmap()?
>>     
>
> Was my first idea as well. I do not understand what is going on there,
> but when I simply exclude #DB from the exception bitmap in real mode,
> the problem pops up again: #GP on the instruction that should raise the #DB.
>
>   

Right.  The docs say that a #DB (like all exceptions) will trap to the 
protected mode handler through the #GP exception.

So, we do need kvm_queue_exception.

>>> /me now wonders if there are not even more exceptions that have to be
>>> forwarded. Right now we catch them all, but I did not find some path
>>> via which actual ones are pushed to the guest.
>>>       
>> Looks like update_exception_bitmap() assumes none.  From a cursory look,
>> seems like #DE (divide error), #DB, #BP (Breakpoint), #OF (overflow),
>> #BR (range check) all need to be passed to the guest.
>>     
>
> Hmm, unless KVM did something to fix up some exception, shouldn't all of
> them be passed?
>   

Yes, they should.  But it can't be done using the exception bitmap.

-- 
error compiling committee.c: too many arguments to function