[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH v2 COLOPre 03/13] libxc/restore: zero ioreq page only one time
On 06/10/2015 06:40 PM, Paul Durrant wrote: >> -----Original Message----- >> From: Wen Congyang [mailto:wency@xxxxxxxxxxxxxx] >> Sent: 10 June 2015 10:06 >> To: Andrew Cooper; Yang Hongyang; xen-devel@xxxxxxxxxxxxx; Paul Durrant >> Cc: Wei Liu; Ian Campbell; yunhong.jiang@xxxxxxxxx; Eddie Dong; >> guijianfeng@xxxxxxxxxxxxxx; rshriram@xxxxxxxxx; Ian Jackson >> Subject: Re: [Xen-devel] [PATCH v2 COLOPre 03/13] libxc/restore: zero ioreq >> page only one time >> >> Cc: Paul Durrant >> >> On 06/10/2015 03:44 PM, Andrew Cooper wrote: >>> On 10/06/2015 06:26, Yang Hongyang wrote: >>>> >>>> >>>> On 06/09/2015 03:30 PM, Andrew Cooper wrote: >>>>> On 09/06/2015 01:59, Yang Hongyang wrote: >>>>>> >>>>>> >>>>>> On 06/08/2015 06:15 PM, Andrew Cooper wrote: >>>>>>> On 08/06/15 10:58, Yang Hongyang wrote: >>>>>>>> >>>>>>>> >>>>>>>> On 06/08/2015 05:46 PM, Andrew Cooper wrote: >>>>>>>>> On 08/06/15 04:43, Yang Hongyang wrote: >>>>>>>>>> ioreq page contains evtchn which will be set when we resume the >>>>>>>>>> secondary vm the first time. The hypervisor will check if the >>>>>>>>>> evtchn is corrupted, so we cannot zero the ioreq page more >>>>>>>>>> than one time. >>>>>>>>>> >>>>>>>>>> The ioreq->state is always STATE_IOREQ_NONE after the vm is >>>>>>>>>> suspended, so it is OK if we only zero it one time. >>>>>>>>>> >>>>>>>>>> Signed-off-by: Yang Hongyang <yanghy@xxxxxxxxxxxxxx> >>>>>>>>>> Signed-off-by: Wen congyang <wency@xxxxxxxxxxxxxx> >>>>>>>>>> CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx> >>>>>>>>> >>>>>>>>> The issue here is that we are running the restore algorithm over a >>>>>>>>> domain which has already been running in Xen for a while. This is a >>>>>>>>> brand new usecase, as far as I am aware. >>>>>>>> >>>>>>>> Exactly. >>>>>>>> >>>>>>>>> >>>>>>>>> Does the qemu process associated with this domain get frozen >>>>>>>>> while the >>>>>>>>> secondary is being reset, or does the process get destroyed and >>>>>>>>> recreated. >>>>>>>> >>>>>>>> What do you mean by reset? do you mean secondary is suspended >> at >>>>>>>> checkpoint? >>>>>>> >>>>>>> Well - at the point that the buffered records are being processed, we >>>>>>> are in the process of resetting the state of the secondary to match >>>>>>> the >>>>>>> primary. >>>>>> >>>>>> Yes, at this point, the qemu process associated with this domain is >>>>>> frozen. >>>>>> the suspend callback will call libxl__qmp_stop(vm_stop() in qemu) to >>>>>> pause >>>>>> qemu. After we processed all records, qemu will be restored with the >>>>>> received >>>>>> state, that's why we add a libxl__qmp_restore(qemu_load_vmstate() >> in >>>>>> qemu) >>>>>> api to restore qemu with received state. Currently in libxl, qemu only >>>>>> start >>>>>> with the received state, there's no api to load received state while >>>>>> qemu is >>>>>> running for a while. >>>>> >>>>> Now I consider this more, it is absolutely wrong to not zero the page >>>>> here. The event channel in the page is not guaranteed to be the same >>>>> between the primary and secondary, >>>> >>>> That's why we don't zero it on secondary. >>> >>> I think you missed my point. Apologies for the double negative. It >>> must, under all circumstances, be zeroed at this point, for safety reasons. >>> >>> The page in question is subject to logdirty just like any other guest >>> pages, which means that if the guest writes to it naturally (i.e. not a >>> Xen or Qemu write, both of whom have magic mappings which are not >>> subject to logdirty), it will be transmitted in the stream. As the >>> event channel could be different, the lack of zeroing it at this point >>> means that the event channel would be wrong as opposed to simply >>> missing. This is a worse position to be in. >> >> The guest should not access this page. I am not sure if the guest can >> access the ioreq page. >> >> But in the exceptional case, the ioreq page is dirtied, and is copied to >> the secondary vm. The ioreq page will contain a wrong event channel, the >> hypervisor will check it: if the event channel is wrong, the guest will >> be crashed. >> >>> >>>> >>>>> and we don't want to unexpectedly >>>>> find a pending/in-flight ioreq. >>>> >>>> ioreq->state is always STATE_IOREQ_NONE after the vm is suspended, >> there >>>> should be no pending/in-flight ioreq at checkpoint. >>> >>> In the common case perhaps, but we must consider the exceptional case. >>> The exceptional case here is some corruption which happens to appear as >>> an in-flight ioreq. >> >> If the state is STATE_IOREQ_NONE, it may be hypervisor's bug. If the >> hypervisor >> has a bug, anything can happen. I think we should trust the hypervisor. >> >>> >>>> >>>>> >>>>> Either qemu needs to take care of re-initialising the event channels >>>>> back to appropriate values, or Xen should tolerate the channels >>>>> disappearing. >>> >>> I still stand by this statement. I believe it is the only safe way of >>> solving the issue you have discovered. >> >> Add a new qemu monitor command to update ioreq page? >> > > If you're attaching to a 'new' VM (i.e one with an updated image) then I > suspect you're going to have to destroy and re-create the ioreq server so > that the shared page gets re-populated with the correct event channels. > Either that or you're going to have to ensure that the page is not part of > restored image and sample the new one that Xen should have set up. I agree with it. I will try to add a new qemu monitor command(or do it when updating qemu's state) to destroy and re-create it. Thanks Wen Congyang > > Paul > > >> Thanks >> Wen Congyang >> >>> >>> ~Andrew >>> . >>> > > . > _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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