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Re: [Qemu-devel] [0/8] RFC: VFIO and guest side IOMMUs, revisited

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [0/8] RFC: VFIO and guest side IOMMUs, revisited
Date: Mon, 13 May 2013 15:30:26 +0200
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Il 13/05/2013 15:13, David Gibson ha scritto:
> On Mon, May 13, 2013 at 02:23:30PM +0200, Paolo Bonzini wrote:
>> Il 13/05/2013 12:54, David Gibson ha scritto:
>>> Specifically the way the iommu is determined from a callback
>>> in the PCIBus means that it won't be assigned for devices under
>>> a PCI-PCI bridge.
>> Right.  I saw the report from Alexey, but I am a bit wary of 
>> touching it because it's not a regression.  In fact there is
>> even a FIXME for it:
>> /* FIXME: inherit memory region from bus creator */
> Uh.. sort of.
>> Perhaps we can make pci_iommu_as a Bus method, where the default
>>  implementation looks up along the chain, and the end of the 
>> recursion is in SysBus or in PCI buses that have set the 
>> callback.
> So, this is complicated by the fact that there are two cases, and 
> they can both be found in existing hardware.
> 1) One is where devices behind the bridge are not visible / 
> differentiable to the IOMMU, and so effectively all their DMAs 
> originate from the bridge device itself.  In this case the correct
>  thing is to give all devices under the bridge the same DMA 
> AddressSpace as the bridge device, as suggested by the FIXME. This
> will be typical behaviour for PCI-E to PCI bridges.
> 2) The other case is where the bridge passes through RIDs, so that 
> the IOMMU can still differentiate devices behind it.  For this 
> case, we really want the hook to be in the host bridge / root bus, 
> and it can make a decision based on the full bus/dev/fn 
> information.  This will be typical for PCI-E to PCI-E bridges (or 
> switches or nexuses or whatever they're usually called for PCI-E). 
> This case will be very important as we start to model newer PCI-E 
> based machines by default, where typically *all* devices are
> behind a logical p2p bridge inside the root complex (but are still 
> differentiable by the Intel IOMMU amongst others).
> I'm not sure at this stage how to properly handle both cases.

Suppose you have a host bridge pci_bus0 and a PCIE->PCIE bridge
pci_bus1.  pci_bus1 does not define a IOMMU callback, pci_bus0 does.

Would it work to use the PCIBus callback provided by pci_bus0, but
invoke it as

    pci_bus0->iommu_fn(pci_bus1, pci_bus0->iommu_opaque, devfn)



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