Re: [Xen-devel] [PATCH-for-4.13] x86/mm: don't needlessly veto migration

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 10/1/19 11:40 AM, Paul Durrant wrote:
>  -----Original Message-----
>> From: George Dunlap <george.dunlap@xxxxxxxxxx>
>> Sent: 01 October 2019 11:34
>> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>; 'Jan Beulich' <jbeulich@xxxxxxxx>
>> Cc: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; Roger Pau Monne 
>> <roger.pau@xxxxxxxxxx>; xen-
>> devel@xxxxxxxxxxxxxxxxxxxx; Juergen Gross <jgross@xxxxxxxx>; Wei Liu 
>> <wl@xxxxxxx>
>> Subject: Re: [Xen-devel] [PATCH-for-4.13] x86/mm: don't needlessly veto 
>> migration
>>
>> On 10/1/19 11:24 AM, Paul Durrant wrote:
>>>> -----Original Message-----
>>>> From: Jan Beulich <jbeulich@xxxxxxxx>
>>>> Sent: 01 October 2019 11:15
>>>> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
>>>> Cc: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; George Dunlap 
>>>> <George.Dunlap@xxxxxxxxxx>; Roger Pau
>>>> Monne <roger.pau@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx; Juergen 
>>>> Gross <jgross@xxxxxxxx>; Wei
>> Liu
>>>> <wl@xxxxxxx>
>>>> Subject: Re: [Xen-devel] [PATCH-for-4.13] x86/mm: don't needlessly veto 
>>>> migration
>>>>
>>>> On 01.10.2019 11:36, Paul Durrant wrote:
>>>>>> -----Original Message-----
>>>>>> From: Jan Beulich <jbeulich@xxxxxxxx>
>>>>>> Sent: 01 October 2019 10:19
>>>>>> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
>>>>>> Cc: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; George Dunlap 
>>>>>> <George.Dunlap@xxxxxxxxxx>; Roger
>> Pau
>>>>>> Monne <roger.pau@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx; Juergen 
>>>>>> Gross <jgross@xxxxxxxx>;
>> Wei
>>>> Liu
>>>>>> <wl@xxxxxxx>
>>>>>> Subject: Re: [Xen-devel] [PATCH-for-4.13] x86/mm: don't needlessly veto 
>>>>>> migration
>>>>>>
>>>>>> On 01.10.2019 10:52, Paul Durrant wrote:
>>>>>>>> -----Original Message-----
>>>>>>>> From: Jan Beulich <jbeulich@xxxxxxxx>
>>>>>>>> Sent: 01 October 2019 09:46
>>>>>>>> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
>>>>>>>> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Andrew Cooper 
>>>>>>>> <Andrew.Cooper3@xxxxxxxxxx>; Roger Pau Monne
>>>>>>>> <roger.pau@xxxxxxxxxx>; George Dunlap <George.Dunlap@xxxxxxxxxx>; 
>>>>>>>> Juergen Gross
>>>> <jgross@xxxxxxxx>;
>>>>>> Wei
>>>>>>>> Liu <wl@xxxxxxx>
>>>>>>>> Subject: Re: [Xen-devel] [PATCH-for-4.13] x86/mm: don't needlessly 
>>>>>>>> veto migration
>>>>>>>>
>>>>>>>> On 01.10.2019 10:28, Paul Durrant wrote:
>>>>>>>>> Now that xl.cfg has an option to explicitly enable IOMMU mappings for 
>>>>>>>>> a
>>>>>>>>> domain, migration may be needlessly vetoed due to the check of
>>>>>>>>> is_iommu_enabled() in paging_log_dirty_enable().
>>>>>>>>> There is actually no need to prevent logdirty from being enabled 
>>>>>>>>> unless
>>>>>>>>> devices are assigned to a domain and that domain is sharing HAP 
>>>>>>>>> mappings
>>>>>>>>> with the IOMMU (in which case disabling write permissions in the P2M 
>>>>>>>>> may
>>>>>>>>> cause DMA faults).
>>>>>>>>
>>>>>>>> But that's taking into account only half of the reason of the
>>>>>>>> exclusion. The other half is that assigned devices may cause pages
>>>>>>>> to be dirtied behind the back of the log-dirty logic.
>>>>>>>
>>>>>>> But that's no reason to veto logdirty. Some devices have drivers (in 
>>>>>>> dom0)
>>>>>>> which can extract DMA dirtying information and set dirty tracking
>>>>>>> information appropriately.
>>>>>>
>>>>>> It still needs a positive identification then: Such drivers should tell
>>>>>> Xen for which specific devices such information is going to be provided.
>>>>>
>>>>> Why does the hypervisor need have the right of veto though? Surely it is
>>>>> the toolstack that should decide whether a VM is migratable in the
>>>>> presence of assigned h/w. Xen need only be concerned with the integrity
>>>>> of the host, which is why the check for ETP sharing remains.
>>>>
>>>> While the tool stack is to decide, the hypervisor is expected to guarantee
>>>> correct data coming back from XEN_DOMCTL_SHADOW_OP_{PEEK,CLEAN}.
>>>
>>> For some definition of 'correct', yes, and I don't think that this change 
>>> violates any definition I
>> can find in the domctl header.
>>>
>>> Note: there are already emulators that will be playing with the dirty map 
>>> on an arbitrary and
>> unsynchronized basis because they are emulating bus mastering h/w.
>>
>> But the question is, do we want the toolstack to have to become an
>> expert in what hardware might have external dirty tracking, and whether
>> such tracking is active?  At the moment that would mean either 1)
>> putting that information inside of libxc, or 2) duplicating it across
>> xapi and libxl, for instance.
> 
> Why not? The toolstack is in charge of migration so why can't it decide 
> whether it is 'safe' or not?

First of all, it's not about what the toolstack can decide; it's what it
knows.  It doesn't currently know anything about the details of devices
themselves or how they relate to other functionality, such as migration.
 Xen knows about specific devices, and could (for instance) special-case
specific device IDs or what-not; and Xen knows that device pass-through
interacts with logdirty.  You're proposing to teach the toolstack about
all of that.

Secondly, you haven't answered the question about duplication.  Where do
you propose to put this functionality?

I'm not 100% opposed to the idea of doing it in the toolstack; defining
"correct" for the logdirty functionality to be "collect changes made by
processors, I'll worry about changes made by devices" is certainly a
reasonable approach, particularly as *somewhere* in the toolstack will
eventually have to actually start these external emulators.

But knowing that emulator X goes with bdf Y is one thing; knowing that
1) devices interact with logdirty, and 2) emulator X knows how to
mitigate that is a different thing.

 -George


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