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

[Paul Durrant <pdurrant@xxxxxxxxx>]

On Wed, 9 Oct 2019 at 09:49, Jan Beulich <jbeulich@xxxxxxxx> wrote:
>
> 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.

Agreed. Given where we are w.r.t. regressions and a release schedule,
I think we need to be pragmatic. Realistically I'm not going get a Xen
dev. environment up and running for maybe a week so I can't work on
this myself at the moment. I am happy for Jan to fix the regressions
and then we can move on after 4.13 is out the door.

  Paul

>
> Jan

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