Re: [Qemu-devel] [RFC PATCH 6/8] memory: introduce AddressSpaceOps

[Peter Xu]

On Mon, May 01, 2017 at 02:58:22PM +1000, David Gibson wrote:
> On Thu, Apr 27, 2017 at 05:34:18PM +0800, Peter Xu wrote:
> > This is something similar to MemoryRegionOps, it's just for address
> > spaces to store arch-specific hooks.
> > 
> > The first hook I would like to introduce is iommu_get().
> > 
> > For systems that have IOMMUs, we will create a special address space per
> > device which is different from system default address space for
> > it (please refer to pci_device_iommu_address_space()). Normally when
> > that happens, there will be one specific IOMMU (or say, translation
> > unit) stands right behind that new address space.
> > 
> > This iommu_get() fetches that guy behind the address space. Here, the
> > guy is defined as IOMMUObject, which is currently a (void *). In the
> > future, maybe we can make it a better definition, but imho it's good
> > enough for now, considering it's arch-dependent.
> > 
> > Signed-off-by: Peter Xu <address@hidden>
> 
> This doesn't make sense to me.  It would be entirely possible for a
> single address space to have different regions mapped by different
> IOMMUs.  Or some regions mapped by IOMMUs and others direct mapped to
> a device or memory block.

Oh, so it's more complicated than I thought... Then, do we really have
existing use case that one device is managed by more than one IOMMU
(on any of the platform)? Frankly speaking I haven't thought about
complicated scenarios like this, or nested IOMMUs yet.

This patch derived from a requirement in virt-svm project (on x86).
Virt-svm needs some notification mechanism for each IOMMU (or say, the
IOMMU that managers the SVM-enabled device). For now, all IOMMU
notifiers are per-memory-region not per-iommu, and that's imho not
what virt-svm wants. Any suggestions?

Thanks,

-- 
Peter Xu