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

From: Peter Xu
Subject: Re: [Qemu-devel] [PATCH v3 3/3] IOMMU: Integrate between VFIO and vIOMMU to support device assignment
Date: Tue, 7 Jun 2016 21:20:08 +0800
User-agent: Mutt/1.5.24 (2015-08-30)

On Mon, Jun 06, 2016 at 12:21:34PM -0600, Alex Williamson wrote:
> I'm not sure why you're so eager to avoid implementing a replay
> callback for VT-d.  What happens when VT-d is not enabled by the
> guest?  vfio/pci.c calls pci_device_iommu_address_space() to get the
> address space for the device, which results in vtd_find_add_as() which
> gives us a unique VTDAddressSpace.as for that device.  With VT-d not in
> use by the guest, when do steps 3-5 occur?  I agree with your reasoning
> when VT-d is enabled, but the BIOS/UEFI does not enable VT-d, so are we
> going to exclude all use cases of an assigned device prior to the guest
> enabling VT-d?

I think I got the point. I failed to consider the case that we can run
IOMMU without enabling it, like BIOS (as you have mentioned), or we
can disable IOMMU in kernel boot parameters (though, iommu=pt should
still follow the case that IOMMU is enabled).

Sounds like a replay callback is a good idea. For Intel version of the
callback: when DMAR is enabled, we can just return directly. when DMAR
is disabled, we just do whatever we need to do region_add() for the
global address_space_memory.

> On that same topic, I'm really dubious that we have the flexibility in
> our memory API to really support an IOMMU like VT-d and the layout of
> having a VTDAddressSpace per device, rather than exposing shared
> address spaces, has implications on the efficiency and locked memory
> requirements for vfio.  In the above case with VT-d disabled, the
> VTDAddressSpace should alias to the system memory AddressSpace and
> dynamically switch to a per device address space when VT-d is enabled.
> AFAICT, we don't have anything resembling that sort of feature, so we
> rely on the IOMMU driver to replay, perhaps even configuring its own
> MemoryListener to IOMMU notifier gateway, which is also something that
> doesn't currently exist.

It sounds more like a notifier for "IOMMU enablement"? The notifier
should be triggered when IOMMU switch between enabled <-> disabled?
When this happens, we rebuild the mapping in some way.

> Additionally, if there are things unique to VT-d, for instance if VT-d
> is enabled and we can rely on the sequence of events you've set forth,
> then yes, the replay mechanism should do nothing.  But only the VT-d
> code can decide that, which implies that vfio always needs to call the
> replay function and a new MemoryRegionIOMMUOps callback needs to decide
> what to do given the current state of the vIOMMU.  Thanks,

Right. Thanks.

-- peterx

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