Re: [Xen-devel] [PATCH v5] x86/emulate: Send vm_event from emulate

[Jan Beulich <JBeulich@xxxxxxxx>]

On 02.07.2019 09:58, Alexandru Stefan ISAILA wrote:
> 
> 
> On 01.07.2019 18:53, Jan Beulich wrote:
>> On 01.07.2019 17:36, Alexandru Stefan ISAILA wrote:
>>> On 01.07.2019 17:55, Jan Beulich wrote:
>>>> On 01.07.2019 16:45, Alexandru Stefan ISAILA wrote:
>>>>> On 01.07.2019 16:13, Jan Beulich wrote:
>>>>>>>>> On 04.06.19 at 13:49, <aisaila@xxxxxxxxxxxxxxx> wrote:
>>>>>>> +    if ( !req.u.mem_access.flags )
>>>>>>> +        return false; /* no violation */
>>>>>>
>>>>>> How is the "false" here (I think this is the one the description talks
>>>>>> about) matching up with the various other ones in the function?
>>>>>
>>>>> There should be no event if there is no access violation. So in this
>>>>> case the emulation is continued as expected.
>>>>
>>>> But this doesn't answer my question: You use "false" as return value
>>>> to indicate different things. Only the one here means "no access
>>>> violation".
>>>
>>> Sorry about that, since this will remain the only return false apart
>>> form the main one (return monitor_traps()), false  = event was not sent
>>> and true = event was sent.
>>>
>>> I understand that you are asking about the scenario when there was a
>>> violation and the event was not sent. Then I can issue a domain_crash()
>>> as that is potentially a big issue.
>>>
>>> I hope I got that correctly.
>>
>> I don't get the impression that you did. I count a total of four
>> "return false" in the function, only one of which explicitly means
>> "no access violation" (others may have that meaning implicitly). Let's
>> take the p2m_get_mem_access() failure case as an example: What I don't
>> understand is why this case and the "no access violation" one are both
>> meant to be treated the same.
> 
> Right, at the moment, false means that emulation should continue and
> true means that emulation should stop. If it is a must that I return
> different errors I will change that in the next version but in the way
> that the code is right now they will be treated the same way.

Again - it's not a requirement. It depends on whether the behavior is
intended to be that way. If it is, it should be clarified in the
description or maybe better in a code comment. But to me, without such
a clarification, it doesn't look like it should be that way.

Jan
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