Re: [RFC v2 12/20] dma-iommu: Implement NESTED_MSI cookie

[Robin Murphy <robin.murphy@arm.com>]

On 2018-10-24 7:44 pm, Auger Eric wrote:
> Hi Robin,
> 
> On 10/24/18 8:02 PM, Robin Murphy wrote:
>> Hi Eric,
>>
>> On 2018-09-18 3:24 pm, Eric Auger wrote:
>>> Up to now, when the type was UNMANAGED, we used to
>>> allocate IOVA pages within a range provided by the user.
>>> This does not work in nested mode.
>>>
>>> If both the host and the guest are exposed with SMMUs, each
>>> would allocate an IOVA. The guest allocates an IOVA (gIOVA)
>>> to map onto the guest MSI doorbell (gDB). The Host allocates
>>> another IOVA (hIOVA) to map onto the physical doorbell (hDB).
>>>
>>> So we end up with 2 unrelated mappings, at S1 and S2:
>>>            S1             S2
>>> gIOVA    ->     gDB
>>>                  hIOVA    ->    hDB
>>>
>>> The PCI device would be programmed with hIOVA.
>>>
>>> iommu_dma_bind_doorbell allows to pass gIOVA/gDB to the host
>>> so that gIOVA can be used by the host instead of re-allocating
>>> a new IOVA. That way the host can create the following nested
>>> mapping:
>>>
>>>            S1           S2
>>> gIOVA    ->    gDB    ->    hDB
>>>
>>> this time, the PCI device will be programmed with the gIOVA MSI
>>> doorbell which is correctly map through the 2 stages.
>>
>> If I'm understanding things correctly, this plus a couple of the
>> preceding patches all add up to a rather involved way of coercing an
>> automatic allocator to only "allocate" predetermined addresses in an
>> entirely known-ahead-of-time manner.
> agreed
>   Given that the guy calling
>> iommu_dma_bind_doorbell() could seemingly just as easily call
>> iommu_map() at that point and not bother with an allocator cookie and
>> all this machinery at all, what am I missing?
> Well iommu_dma_map_msi_msg() gets called and is part of this existing
> MSI mapping machinery. If we do not do anything this function allocates
> an hIOVA that is not involved in any nested setup. So either we coerce
> the allocator in place (which is what this series does) or we unplug the
> allocator to replace this latter with a simple S2 mapping, as you
> suggest, ie. iommu_map(gDB, hDB). Assuming we unplug the allocator, the
> guy who actually calls  iommu_dma_bind_doorbell() knows gDB but does not
> know hDB. So I don't really get how we can simplify things.

OK, there's what I was missing :D

But that then seems to reveal a somewhat bigger problem - if the callers 
are simply registering IPAs, and relying on the ITS driver to grab an 
entry and fill in a PA later, then how does either one know *which* PA 
is supposed to belong to a given IPA in the case where you have multiple 
devices with different ITS targets assigned to the same guest? (and if 
it's possible to assume a guest will use per-device stage 1 mappings and 
present it with a single vITS backed by multiple pITSes, I think things 
start breaking even harder.)

Other than allowing arbitrary disjoint IOVA pages, I'm not sure this 
really works any differently from the existing MSI cookie now that I 
look more closely :/

Robin.