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Re: [Xen-devel] [PATCH v7] x86/emulate: Send vm_event from emulate
- To: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>
- From: Jan Beulich <JBeulich@xxxxxxxx>
- Date: Fri, 19 Jul 2019 13:38:54 +0000
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- Cc: PetreOvidiu PIRCALABU <ppircalabu@xxxxxxxxxxxxxxx>, "tamas@xxxxxxxxxxxxx" <tamas@xxxxxxxxxxxxx>, "wl@xxxxxxx" <wl@xxxxxxx>, "george.dunlap@xxxxxxxxxxxxx" <george.dunlap@xxxxxxxxxxxxx>, "andrew.cooper3@xxxxxxxxxx" <andrew.cooper3@xxxxxxxxxx>, "paul.durrant@xxxxxxxxxx" <paul.durrant@xxxxxxxxxx>, Alexandru Stefan ISAILA <aisaila@xxxxxxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, "roger.pau@xxxxxxxxxx" <roger.pau@xxxxxxxxxx>
- Delivery-date: Fri, 19 Jul 2019 13:39:53 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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- Thread-topic: [PATCH v7] x86/emulate: Send vm_event from emulate
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).
Jan
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