Re: [Xen-devel] [PATCH v7] x86/emulate: Send vm_event from emulate

[Alexandru Stefan ISAILA <aisaila@xxxxxxxxxxxxxxx>]

On 19.07.2019 17:23, Razvan Cojocaru wrote:
> On 7/19/19 4:38 PM, Jan Beulich wrote:
>> On 19.07.2019 15:30, Razvan Cojocaru wrote:
>>> On 7/19/19 4:18 PM, Jan Beulich wrote:
>>>> On 19.07.2019 14:34, Alexandru Stefan ISAILA wrote:
>>>>> On 18.07.2019 15:58, Jan Beulich wrote:
>>>>>> On 03.07.2019 12:56, Alexandru Stefan ISAILA wrote:
>>>>>>> A/D bit writes (on page walks) can be considered benign by an 
>>>>>>> introspection
>>>>>>> agent, so receiving vm_events for them is a pessimization. We try 
>>>>>>> here to
>>>>>>> optimize by fitering these events out.
>>>>>>
>>>>>> But you add the sending of more events - how does "filter out" match
>>>>>> the actual implementation?
>>>>>
>>>>> The events are send only if there is a mem access violation 
>>>>> therefore we
>>>>> are filtering and only sending the events that are interesting to
>>>>> introspection.
>>>>
>>>> Where is it that you prevent any event from being sent? As said,
>>>> reading the patch I only see new sending sites to get added.
>>>
>>> If we don't emulate, we would receive the page-walk-generated events
>>> _and_ the touching-the-page-the-instruction-is-touching events.
>>
>> Since the patch here alters emulation paths only, how do you know
>> whether to emulate? In order to not receive undue events it would
>> seem to me that you'd first have to intercept the guest on insns
>> of interest ... Overall I think that the patch description, while
>> it has improved, is still lacking sufficient information for a
>> person like me (not knowing much about your monitor tools) to be
>> able to sensibly review this (which includes understanding the
>> precise scenario you want to improve).
> 
> If the hardware exits because of an EPT fault caused by a page walk, we 
> end up in p2m_mem_access_check(), at which point we need to decide if we 
> want to send out a vm_event or not.
> 
> If we were to send out this vm_event, and it would then be magically 
> treated so that we get to actually run the instruction at RIP, said 
> instruction might also hit a protected page and provoke a vm_event.
> 
> Now, if npfec.kind != npfec_kind_with_gla, then we're in the page walk 
> case, and so in this case only, and only if 
> d->arch.monitor.inguest_pagefault_disabled is true, we would choose to 
> do this emulation trick: emulate _the_page_walk_ while ignoring the EPT, 
> but don't ignore the EPT for the emulation of the actual instruction.
> 
> So where in the first case we would have 2 EPT events, in the second we 
> only have one (or if the instruction at RIP does not trigger an EPT 
> event, we would have 1 event in the first case, and none in the second).
> Hence the filtering mentioned.
> 
> So to answer your question: "how do you know whether to emulate", we do 
> so only if npfec.kind != npfec_kind_with_gla && 
> d->arch.monitor.inguest_pagefault_disabled.
> 
> I hope this clears it up somewhat.
> 

To summarize the changes needed for the next version, apart from the 
code changes, is the description good or do I have to add something else?

Thanks,
Alex
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