Re: [PATCH v2 3/3] x86/ioreq server: Add HVMOP to map guest ram with p2m_ioreq_server to an ioreq server

["Yu, Zhang" <yu.c.zhang@linux.intel.com>]

On 4/19/2016 12:37 PM, Tian, Kevin wrote:
>> From: Yu, Zhang [mailto:yu.c.zhang@linux.intel.com]
>> Sent: Thursday, April 14, 2016 6:45 PM
>>
>> On 4/11/2016 7:15 PM, Yu, Zhang wrote:
>>>
>>>
>>> On 4/8/2016 7:01 PM, George Dunlap wrote:
>>>> On 08/04/16 11:10, Yu, Zhang wrote:
>>>> [snip]
>>>>> BTW, I noticed your reply has not be CCed to mailing list, and I also
>>>>> wonder if we should raise this last question in community?
>>>>
>>>> Oops -- that was a mistake on my part.  :-)  I appreciate the
>>>> discretion; just so you know in the future, if I'm purposely changing
>>>> the CC list (removing xen-devel and/or adding extra people), I'll almost
>>>> always say so at the top of the mail.
>>>>
>>>>>> And then of course there's the p2m_ioreq_server -> p2m_ram_logdirty
>>>>>> transition -- I assume that live migration is incompatible with this
>>>>>> functionality?  Is there anything that prevents a live migration from
>>>>>> being started when there are outstanding p2m_ioreq_server entries?
>>>>>>
>>>>>
>>>>> Another good question, and the answer is unfortunately yes. :-)
>>>>>
>>>>> If live migration happens during the normal emulation process, entries
>>>>> marked with p2m_ioreq_server will be changed to p2m_log_dirty in
>>>>> resolve_misconfig(), and later write operations will change them to
>>>>> p2m_ram_rw, thereafter these pages can not be forwarded to device model.
>>>>>   From this point of view, this functionality is incompatible with live
>>>>> migration.
>>>>>
>>>>> But for XenGT, I think this is acceptable, because, if live migration
>>>>> is to be supported in the future, intervention from backend device
>>>>> model will be necessary. At that time, we can guarantee from the device
>>>>> model side that there's no outdated p2m_ioreq_server entries, hence no
>>>>> need to reset the p2m type back to p2m_ram_rw(and do not include
>>>>> p2m_ioreq_server in the P2M_CHANGEABLE_TYPES). By "outdated", I mean
>>>>> when an ioreq server is detached from p2m_ioreq_server, or before an
>>>>> ioreq server is attached to this type, entries marked with
>>>>> p2m_ioreq_server should be regarded as outdated.
>>>>>
>>>>> Is this acceptible to you? Any suggestions?
>>>>
>>>> So the question is, as of this series, what happens if someone tries to
>>>> initiate a live migration while there are outstanding p2m_ioreq_server
>>>> entries?
>>>>
>>>> If the answer is "the ioreq server suddenly loses all control of the
>>>> memory", that's something that needs to be changed.
>>>>
>>>
>>> Sorry, for this patch series, I'm afraid the above description is the
>>> answer.
>>>
>>> Besides, I find it's hard to change current code to both support the
>>> deferred resetting of p2m_ioreq_server and the live migration at the
>>> same time. One reason is that a page with p2m_ioreq_server behaves
>>> differently in different situations.
>>>
>>> My assumption of XenGT is that, for live migration to work, the device
>>> model should guarantee there's no outstanding p2m_ioreq_server pages
>>> in hypervisor(no need to use the deferred recalculation), and it is our
>>> device model who should be responsible for the copying of the write
>>> protected guest pages later.
>>>
>>> And another solution I can think of: when unmapping the ioreq server,
>>> we walk the p2m table and reset entries with p2m_ioreq_server back
>>> directly, instead of deferring the reset. And of course, this means
>>> performance impact. But since the mapping and unmapping of an ioreq
>>> server is not a frequent one, the performance penalty may be acceptable.
>>> How do you think about this approach?
>>>
>>
>> George, sorry to bother you. Any comments on above option? :)
>>
>> Another choice might be to let live migration fail if there's
>> outstanding p2m_ioreq_server entries. But I'm not quite inclined to do
>> so, because:
>> 1> I'd still like to keep live migration feature for XenGT.
>> 2> Not easy to know if there's outstanding p2m_ioreq_server entries. I
>> mean, since p2m type change is not only triggered by hypercall, to keep
>> a counter for remaining p2m_ioreq_server entries means a lot code
>> changes;
>>
>> Besides, I wonder whether the requirement to reset the p2m_ioreq_server
>> is indispensable, could we let the device model side to be responsible
>> for this? The worst case I can imagine for device model failing to do
>> so is that operations of a gfn might be delivered to a wrong device
>> model. I'm not clear what kind of damage would this cause to the
>> hypervisor or other VM.
>>
>> Does any other maintainers have any suggestions?
>> Thanks in advance! :)
>
> I'm not sure how above is working. In pre-copy phase (where logdirty
> is concerned), the device model is still actively serving requests from
> guest, including initiating new write-protection requests. How can you
> guarantee draining of outstanding p2m_ioreq_server entries w/o
> actually freezing device model (while freezing device model means guest
> driver might be blocked with random errors)?
>

You are right, and I'm not suggesting to clear the p2m_ioreq_server
entries when live migration happens. My suggestion is that either we
guarantee there is no outstanding p2m_ioreq_server entries right after
the ioreq server is unbounded, or do not support live migration for
now.  :)

B.R.
Yu

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