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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.
I also wonder what interface I would have forgot about that would allow
such a driver to propagate dirtying information in the first place:
XEN_DMOP_modified_memory is for emulators only aiui, and does not provide
a means for Xen to actively query dirtied state (or request updating
thereof) of pages owned by a domain (as would be needed at least on the
XEN_DOMCTL_SHADOW_LOGDIRTY_FINAL invocation).

Jan

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel