From: Alexandru Stefan ISAILA <aisaila@bitdefender.com>
To: Razvan Cojocaru <rcojocaru@bitdefender.com>,
Jan Beulich <JBeulich@suse.com>
Cc: Petre Ovidiu PIRCALABU <ppircalabu@bitdefender.com>,
"tamas@tklengyel.com" <tamas@tklengyel.com>,
"wl@xen.org" <wl@xen.org>,
"george.dunlap@eu.citrix.com" <george.dunlap@eu.citrix.com>,
"andrew.cooper3@citrix.com" <andrew.cooper3@citrix.com>,
"paul.durrant@citrix.com" <paul.durrant@citrix.com>,
"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
"roger.pau@citrix.com" <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH v7] x86/emulate: Send vm_event from emulate
Date: Mon, 29 Jul 2019 08:12:05 +0000 [thread overview]
Message-ID: <9a509cd8-d91a-036d-f949-f1c5d54059d6@bitdefender.com> (raw)
In-Reply-To: <ded7b153-d2cd-15e1-93cc-9aaab2a06d95@bitdefender.com>
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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next prev parent reply other threads:[~2019-07-29 8:12 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-03 10:56 [Xen-devel] [PATCH v7] x86/emulate: Send vm_event from emulate Alexandru Stefan ISAILA
2019-07-11 17:13 ` Tamas K Lengyel
2019-07-12 1:28 ` Jan Beulich
2019-07-15 8:52 ` Alexandru Stefan ISAILA
2019-07-18 12:58 ` Jan Beulich
2019-07-19 12:34 ` Alexandru Stefan ISAILA
2019-07-19 13:18 ` Jan Beulich
2019-07-19 13:30 ` Razvan Cojocaru
2019-07-19 13:38 ` Jan Beulich
2019-07-19 14:23 ` Razvan Cojocaru
2019-07-29 8:12 ` Alexandru Stefan ISAILA [this message]
2019-07-29 11:30 ` Jan Beulich
2019-07-22 7:51 ` Alexandru Stefan ISAILA
2019-07-30 12:21 ` Alexandru Stefan ISAILA
2019-07-30 13:27 ` Jan Beulich
2019-07-30 14:12 ` Alexandru Stefan ISAILA
2019-07-30 14:54 ` Jan Beulich
2019-07-30 15:28 ` Alexandru Stefan ISAILA
2019-08-20 20:11 ` Andrew Cooper
2019-08-27 8:26 ` Jan Beulich
2019-09-02 14:36 ` Alexandru Stefan ISAILA
2019-09-02 14:59 ` Jan Beulich
2019-07-23 8:17 ` Alexandru Stefan ISAILA
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