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Re: [Xen-devel] [PATCH-for-4.13 v2] x86/mm: don't needlessly veto migration
- To: Jan Beulich <jbeulich@xxxxxxxx>, Paul Durrant <paul.durrant@xxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Wed, 2 Oct 2019 10:10:01 +0100
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- Cc: George Dunlap <george.dunlap@xxxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, Juergen Gross <jgross@xxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Wed, 02 Oct 2019 09:10:12 +0000
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On 02/10/2019 09:40, Jan Beulich wrote:
> On 01.10.2019 17:11, 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). It is quite possible that some assigned devices may
>> provide information about which pages may have been dirtied by DMA via
>> an API exported by their managing emulator. Thus Xen's logdirty map is only
>> one source of information that may be available to the toolstack when
>> performing a migration and hence it is the toolstack that is best placed
>> to decide under what circumstances it can be performed, not the hypervisor.
> While I'm happy about the extended description, it's still written in
> a way suggesting that this is the only possible way of viewing things.
> As expressed by George and me, putting the hypervisor in a position to
> be able to judge is at least an alternative worth considering.
No, for exactly the same reason as I'm purging the disable_migrate flag.
This is totally backwards thinking, because the check is in the wrong place.
There really are cases where the toolstack, *and only* the toolstack is
in a position to determine migration safety. When it comes to
disable_migrate, the area under argument is the ITSC flag, which *is*
safe to offer on migrate for viridian guests which are known to use
reference_tsc, or if the destination hardware supports tsc scaling.
(Hilariously, nothing, not even the toolstack, prohibits migration based
on Xen's no-migrate flag, because its a write-only field which can't be
retrieved by the tools.)
The two options are:
1) New hypercall,
DOMCTL_the_toolstack_knows_wtf_its_doing_so_let_the_doimain_migrate,
which disables the vetos,
or
2) Delete the erroneous vetos, and trust that the toolstack knows what
it is doing, and will only initiate a migrate in safe situations.
Option 2 has the safety checks perfomed at the level which is actually
capable of calculating the results correctly.
One of these options is substantially less bone-headed than the other.
~Andrew
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