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Re: [Qemu-devel] [PATCH v3 3/3] IOMMU: Integrate between VFIO and vIOMMU

From: Aviv B.D.
Subject: Re: [Qemu-devel] [PATCH v3 3/3] IOMMU: Integrate between VFIO and vIOMMU to support device assignment
Date: Sat, 28 May 2016 16:10:55 +0000

On Sat, May 28, 2016 at 7:02 PM Alex Williamson <address@hidden>

> On Sat, 28 May 2016 10:52:58 +0000
> "Aviv B.D." <address@hidden> wrote:
> > Hi,
> > Your idea to search the relevent VTDAddressSpace and call it's notifier
> > will
> > probably work. Next week I'll try to implement it (for now with the
> costly
> > scan
> > of each context).
> I think an optimization we can make is to use pci_for_each_bus() and
> pci_for_each_device() to scan only context entries where devices are
> present.  Then for each context entry, retrieve the DID, if it matches
> the invalidation domain_id, retrieve the VTDAddressSpace and perform a
> memory_region_notify_iommu() using VTDAddressSpace.iommu.  Still
> horribly inefficient, but an improvement over walking all context
> entries and avoids gratuitous callbacks between unrelated drivers in

Thanks for the references on how I can do it. :)

> Overall, I have very little faith that this will be the only change
> required to make this work though.  For instance, if a device is added
> or removed from a domain, where is that accounted for?  Ideally this
> should trigger the region_add/region_del listener callbacks, but I
> don't see how that works with how VT-d creates a fixed VTDAddressSpace
> per device, and in fact how our QEMU memory model doesn't allow the
> address space of a device to be dynamically aliased against other
> address spaces or really changed at all.
> > I still not sure if populating the MemoryRegion will suffice for hot plug
> > vfio
> > device but i'll try to look into it.
> >
> > As far as I understand the memory_region_iommu_replay function, it still
> > scans
> > the whole 64bit address space, and therefore may hang the VM for a long
> > time.
> Then we need to fix that problem, one option might be to make a replay
> callback on MemoryRegionIOMMUOps that walks the page tables for a given
> context entry rather than blindly traversing a 64bit address space.  We
> can't simply ignore the issue by #ifdef'ing out the code.  I suspect
> there's a lot more involved to make VT-d interact properly with a
> physical device than what's been proposed so far.  At every
> invalidation, we need to figure out what's changed and update the host
> mappings.  We also need better, more dynamic address space management
> to make the virtual hardware reflect physical hardware when we enable
> things like passthrough mode or have multiple devices sharing an iommu
> domain.  I think we're just barely scratching the surface here.  Thanks,
> Alex

I agree with you regarding hotplug, therefore I only ifdef this code out
and didn't
delete it. With the call to memory_region_iommu_replay QEMU hangs on startup
with a very long loop that prevent any device assignment  with vIOMMU

I'm hoping not to enlarge the scope of this patch to include hotplug device
with iommu enabled.


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