Re: [Xen-devel] [PATCH v6] x86/monitor: Notify monitor if an emulation fails.

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 07/18/2017 01:20 PM, Razvan Cojocaru wrote:
> On 07/18/2017 01:09 PM, Andrew Cooper wrote:
>> On 18/07/17 10:37, Petre Pircalabu wrote:
>>> If case of a vm_event with the emulate_flags set, if the instruction
>>> cannot be emulated, the monitor should be notified instead of directly
>>> injecting a hw exception.
>>> This behavior can be used to re-execute an instruction not supported by
>>> the emulator using the real processor (e.g. altp2m) instead of just
>>> crashing.
>>>
>>> Signed-off-by: Petre Pircalabu <ppircalabu@xxxxxxxxxxxxxxx>
>>
>> There are many situations which end up failing an emulation with
>> UNHANDLEABLE.
>>
>> What exact scenario are you looking to catch?  Is it just instructions
>> which aren't implemented in the emulator?
> 
> Instructions that are not implemented in the emulator are our main use
> case for this, yes. In which case, we'd like a chance to be able to
> single-step them (using altp2m), so that the guest will continue to run
> even with an incomplete emulator.
> 
> We don't care about instructions that would have failed to run in both
> scenarios (emulated or single-stepped). I'm not sure if there are other
> cases in which an instruction, although supported by the emulator, would
> fail emulation but pass single-stepping.

Is there further action required on our part the get this patch
commited? FWIW, while not ideal, from our perspective trying to
single-step an instruction that was UNHANDLEABLE when attempting to
emulate it is acceptable (even if UNHANDLEABLE doesn't mean that the
instruction is not supported by the emulator). At worst it won't run
when single-stepped either, and that'll be that.


Thanks,
Razvan

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