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Re: [Qemu-devel] [PATCH v6 5/7] vfio: Introduce VFIO address spaces

From: Alex Williamson
Subject: Re: [Qemu-devel] [PATCH v6 5/7] vfio: Introduce VFIO address spaces
Date: Tue, 27 May 2014 13:37:15 -0600

On Sun, 2014-05-25 at 23:36 +1000, David Gibson wrote:
> On Sun, May 25, 2014 at 12:16:20PM +0200, Alexander Graf wrote:
> > 
> > On 24.05.14 05:12, Alexey Kardashevskiy wrote:
> > >On 05/24/2014 07:15 AM, Alexander Graf wrote:
> > >>On 23.05.14 18:16, Alexey Kardashevskiy wrote:
> > >>>On 05/23/2014 10:05 PM, Alexander Graf wrote:
> > >>>>On 23.05.14 14:03, Alexey Kardashevskiy wrote:
> > >>>>>On 05/23/2014 09:28 PM, Alexander Graf wrote:
> > >>>>>>On 23.05.14 06:59, Alexey Kardashevskiy wrote:
> > >>>>>>>From: David Gibson <address@hidden>
> > >>>>>>>
> > >>>>>>>The only model so far supported for VFIO passthrough devices is the
> > >>>>>>>model
> > >>>>>>>usually used on x86, where all of the guest's RAM is mapped into the
> > >>>>>>>(host) IOMMU and there is no IOMMU visible in the guest.
> > >>>>>>>
> > >>>>>>>This patch begins to relax this model, introducing the notion of a
> > >>>>>>>VFIOAddressSpace.  This represents a logical DMA address space which
> > >>>>>>>will
> > >>>>>>>be visible to one or more VFIO devices by appropriate mapping in the
> > >>>>>>>(host)
> > >>>>>>>IOMMU.  Thus the currently global list of containers becomes local to
> > >>>>>>>a VFIOAddressSpace, and we verify that we don't attempt to add a VFIO
> > >>>>>>>group to multiple address spaces.
> > >>>>>>>
> > >>>>>>>For now, only one VFIOAddressSpace is created and used, 
> > >>>>>>>corresponding to
> > >>>>>>>main system memory, that will change in future patches.
> > >>>>>>>
> > >>>>>>>Signed-off-by: David Gibson <address@hidden>
> > >>>>>>>Signed-off-by: Alexey Kardashevskiy <address@hidden>
> > >>>>>>Don't we already have a DMA address space in the PCI bus? We could
> > >>>>>>just use
> > >>>>>>that one instead, no?
> > >>>>>I do not know about x86, but for spapr that VFIOAddressSpace is nothing
> > >>>>>but
> > >>>>>wrapper around an AddressSpace from the SPAPR PHB.
> > >>>>So why do we need that wrapper? Can't we just use the PHB's 
> > >>>>AddressSpace?
> > >>>>There's a good chance I'm not grasping something here :).
> > >>>We cannot attach VFIO containers (aka "groups" or "PEs" for spapr) to
> > >>>AddressSpace, there is nothing like that in AddressSpace/MemoryRegion API
> > >>>as this container thing is local to VFIO.
> > >>Ok, please explain how this AddressSpace is different from the VFIO
> > >>device's parent's IOMMU DMA AddressSpace and why we need it.
> > >Nothing special. We attach group to address space by trying to add a group
> > >to every container in that address space. If it fails, we create a new
> > >container, put new group into it and attach container to the VFIO address
> > >space. The point here is we attach group to address space.
> > >
> > >We could still have a global containers list and when adding a group, loop
> > >through the global list of containers and look at the AS they are attached
> > >to but the logical structure AS->container->group->device remains the same.
> > 
> > I honestly still have no idea what all of this is doing and why we can't
> > model it with PCI buses' IOMMU ASs. Alex, do you grasp it?
> It's a while since I looked at this, so I may be forgetting.
> But, I think it's simply a place to store the VFIO-specific
> per-address-space information.

Right, note that we're not actually creating any new AddressSpaces here,
we're simply re-organizing our container list based on which address
space it maps.  On x86 this relocates the global VFIO container list
under a single VFIOAddressSpace for address_space_memory.  On SPAPR each
device can potentially be in it's own pci_device_iommu_address_space
since IOMMUs are exposed to the guest.  A container maps one or more
groups to an AddressSpace where a group may have one or more devices.
If the group for a device already belongs to a container in one
AddressSpace, we cannot map that device to a different AddressSpace.
This is just a re-org to make that possible.  Thanks,


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