On 29.07.2019 10:12, Alexandru Stefan ISAILA wrote: > > > 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?
As said in a prior reply, Razvan's explanation has helped. I don't think though that it's suitable as a patch description without some adjustments. I further seem to recall that I had asked for a concrete example to be laid out in the description, highlighting what exactly in the overall flow your patch means to change. Jan _______________________________________________ Xen-devel mailing list Xen-devel@lists.xenproject.org https://lists.xenproject.org/mailman/listinfo/xen-devel
