Re: [Qemu-devel] [PATCH v4 2/3] memory: introduce IOMMUOps.notify_flag_changed

[David Gibson <david@gibson.dropbear.id.au>]

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On Wed, Sep 14, 2016 at 03:49:41PM +0800, Peter Xu wrote:
> 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".

Huh? I have no idea what you mean by that.

Power can support notifying both map and unmap events just fine - but
in order to provide *any* notifications, it has to disable KVM
acceleration of the guest-side IOMMU (otherwise qemu won't know about
any changes to the IOMMU state).

So the change you you suggested before to != IOMMU_NOTIFIER_NONE is
exactly correct, anything else is not.

> 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.

Hmm.. I really get the feeling you're confusing constraints of the
guest side with constraints of the host side.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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