Re: [Qemu-devel] [PATCH 2/3] memory: add iommu_notify_flag

From: David Gibson <david@gibson.dropbear.id.au>
To: Peter Xu <peterx@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	mst@redhat.com, jasowang@redhat.com, qemu-devel@nongnu.org,
	cornelia.huck@de.ibm.com, alex.williamson@redhat.com,
	wexu@redhat.com, vkaplans@redhat.com, dgibson@redhat.com
Subject: Re: [Qemu-devel] [PATCH 2/3] memory: add iommu_notify_flag
Date: Wed, 7 Sep 2016 16:41:54 +1000	[thread overview]
Message-ID: <20160907064154.GR2780@voom.fritz.box> (raw)
In-Reply-To: <20160907063419.GI21051@pxdev.xzpeter.org>

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On Wed, Sep 07, 2016 at 02:34:19PM +0800, Peter Xu wrote:
> On Wed, Sep 07, 2016 at 03:44:19PM +1000, David Gibson wrote:
> > > For "CHANGE", it sounds like a unmap() + a map(). However I'd say
> > > "ADDITION" is nowhere better...
> > 
> > Right.. this brings up a good point.
> > 
> > Changing a mapping (i.e. overwriting an existing mapping with a
> > different one) would also need notification, even on x86, no?  Since
> > it implicitly invalidates the previous mapping.
> > 
> > I'm guessing the guest will avoid this by always unmapping before it
> > maps.  We still need to consider this possibility when designing the
> > notifier interface though.
> > 
> > It seems the real notification triggers here are:
> >     * map - something is mapped which previously wasn't
> >     * unmap - something is no longer mapped which was before
> > 
> > Note that whether the second needs to be triggered depends on the
> > *previous* state of that IOBA range, *not* on the permissions of the
> > new mapping (if any).
> > 
> > A "change" - replacing one mapping with another should count as both a
> > "map" and "unmap" event.
> 
> Yeah... For MAP/UNMAP, it is strange in another way: e.g. for vhost,
> it doesn't care about map/unmap, it cares about invalidated cache.

I think caring about invalidated cache *is* caring about unmap.  It
doesn't matter whether the new mapping is something or nothing - if
the old mapping is no longer valid, you need to invalidate the cache,
yes?

> So
> IIUC this is a question about "naming" but not the implementations...
> I suppose it is really a matter of taste, and both work for me (either
> INVALIDATION/CHANGE or UNMAP/MAP).

No.. it is a question of implementation.  My point is that I don't
think the new permission is sufficient information to let you know if
a notification is necessary.  You need to know if there was an
existing mapping at that IOBA.

-- 
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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