Re: [Xen-devel] [PATCH v3] x86/mm: don't needlessly veto migration

[Jan Beulich <jbeulich@xxxxxxxx>]

On 08.10.2019 18:38, Andrew Cooper wrote:
> On 08/10/2019 17:10, Jan Beulich wrote:
>> From: Paul Durrant <paul.durrant@xxxxxxxxxx>
>>
>> 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.
>>
>> NOTE: While in the neighbourhood, the bool_t parameter type in
>>       paging_log_dirty_enable() is replaced with a bool and the format
>>       of the comment in assign_device() is fixed.
>>
>> Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> Release-acked-by: Juergen Gross <jgross@xxxxxxxx>
> 
> Seriously FFS.  Why am I having to repeat myself?  What if any way
> unclear on the previous threads?
> 
> NACK NACK NACK.  Xen is, and has always been, the wrong place to have
> any logic, because IT DOESN'T HAVE ENOUGH INFORMATION TO MAKE THE
> DECISION CORRECTLY.
> 
> The toolstack does.
> 
> Therefore, the toolstack is the only level capable decide whether it is
> safe to migration/suspend/resume/checkpoint the VM.
> 
> If I have to write the patches myself, I will, but this patch in this
> form is frankly unacceptable.

You're kidding, aren't you? By taking only part of Paul's original
patch, we should be able to get rid of two of the current osstest
reported regressions. At the same time this _does not_ exclude an
incremental subsequent patch to also add the other half (see my
reply to him yesterday suggesting this split). The two steps
shouldn't have been merged into a single patch anyway imo: The
part here fixes a regression, while the other part changes original
behavior, and continues to be (irrespective of your wording, which
once again suggests that in certain cases you aren't willing to
tolerate thinking that's different from yours) controversial.

If it helps, I can change the title (and perhaps description) to
make it look less like the original patch, and to put focus on the
regression. I just didn't want to massage it more than absolutely
needed.

Jan

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