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Date: Wed, 14 Sep 2016 15:49:41 +0800
From: Peter Xu <peterx@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v4 2/3] memory: introduce
 IOMMUOps.notify_flag_changed
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To: David Gibson <david@gibson.dropbear.id.au>
Cc: qemu-devel@nongnu.org, mst@redhat.com, jasowang@redhat.com, vkaplans@redhat.com, alex.williamson@redhat.com, wexu@redhat.com, pbonzini@redhat.com, cornelia.huck@de.ibm.com, dgibson@redhat.com

On Wed, Sep 14, 2016 at 05:22:40PM +1000, David Gibson wrote:
> On Wed, Sep 14, 2016 at 03:12:43PM +0800, Peter Xu wrote:
> > On Wed, Sep 14, 2016 at 03:55:28PM +1000, David Gibson wrote:
> > 
> > [...]
> > 
> > > > -static void vtd_iommu_notify_started(MemoryRegion *iommu)
> > > > +static void vtd_iommu_notify_flag_changed(MemoryRegion *iommu,
> > > > +                                          IOMMUNotifierFlag old,
> > > > +                                          IOMMUNotifierFlag new)
> > > >  {
> > > >      VTDAddressSpace *vtd_as = container_of(iommu, VTDAddressSpace, iommu);
> > > 
> > > Shouldn't this have a sanity check that the new flags doesn't include
> > > MAP actions?
> > 
> > See your r-b for patch 3, thanks! So skipping this one.
> > 
> > [...]
> > 
> > > > +static void spapr_tce_notify_flag_changed(MemoryRegion *iommu,
> > > > +                                          IOMMUNotifierFlag old,
> > > > +                                          IOMMUNotifierFlag new)
> > > > +{
> > > > +    if (old == IOMMU_NOTIFIER_NONE && new == IOMMU_NOTIFIER_ALL) {
> > > > +        spapr_tce_notify_started(iommu);
> > > > +    } else if (old == IOMMU_NOTIFIER_ALL && new == IOMMU_NOTIFIER_NONE) {
> > > > +        spapr_tce_notify_stopped(iommu);
> > > > +    }
> > > 
> > > This is wrong.  We need to do the notify_start and stop actions if
> > > *any* bits are set in the new/old flags, not just if all of them are
> > > set.
> > 
> > Power should need both, right? I can switch all
> > 
> >   "== IOMMU_NOTIFIER_ALL"
> > 
> > into:
> > 
> >   "!= IOMMU_NOTIFIER_NONE"
> 
> Yes, that should be right.
> 
> > in the next version if you like, but AFAICT they are totally the
> > same.
> 
> Again, only if you assume things about how the notifiers will be used,
> which is not a good look when designing an interface.

This should not be related to the interface at all?

I was based on the assumption that "Power cannot support either one of
MAP/UNMAP, but only if both exist". To be more specific, we possibly
can have this at the beginning of flags_changed():

  assert(old == IOMMU_NOTIFIER_NONE || old == IOMMU_NOTIFIER_ALL);
  assert(new == IOMMU_NOTIFIER_NONE || new == IOMMU_NOTIFIER_ALL);

To make sure nothing will go wrong.

Thanks,

-- peterx