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

[Peter Xu]

On Wed, Sep 07, 2016 at 04:41:54PM +1000, David Gibson wrote:
> 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 the old mapping is no longer valid, you need to invalidate the cache,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?

Yes, I think these two are exactly the same in implementation (vhost
needs UNMAP events of course). So that's why I called it "a naming
issue". :)

> 
> > 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.

My understanding is that we don't need to know that. Because IIUC
there are only map_page() and unmap_page() in guest IOMMU driver
(please check dma_map_ops in kernel). There is no chance for anyone to
"change" the content of the mapping, unless it calls unmap_page() then
with a map_page(). In that case, we'll have two IOTLB invalidation
requests.

Please kindly correct me if I am wrong.

Thanks,

-- peterx