Re: [Xen-devel] [PATCH v2 COLOPre 03/13] libxc/restore: zero ioreq page only one time

[Wen Congyang <wency@xxxxxxxxxxxxxx>]

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
>>> .
>>>
> 
> .
> 


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