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

From: David Gibson
Subject: Re: [Qemu-devel] [PATCH v6 5/7] vfio: Introduce VFIO address spaces
Date: Sun, 25 May 2014 23:36:43 +1000
User-agent: Mutt/1.5.23 (2014-03-12)

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.

David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!

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