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Subject: RE: [RFC 2/3] virtio-iommu: device probing and operations

> From: Jean-Philippe Brucker [mailto:jean-philippe.brucker@arm.com]
> Sent: Tuesday, August 22, 2017 10:19 PM
> On 22/08/17 07:24, Tian, Kevin wrote:
> >>> (sorry to pick up this old thread, as the .tex one is not good for review
> >>> and this thread provides necessary background for IOASID).
> >>>
> >>> Hi, Jean,
> >>>
> >>> I'd like to hear more clarification regarding the relationship between
> >>> IOASID and PASID. When reading back above explanation, it looks
> >>> confusing to me now (though I might get the meaning months ago :/).
> >>> At least Intel VT-d only understands PASID (or you can think IOASID
> >>> =PASID). There is no such layered address space concept. Then for
> >>> map/unmap type request, do you intend to steal some PASIDs for
> >>> that purpose on such architecture (since IOASID is a mandatory field
> >>> in map/unmap request)?
> >>
> >> IOASID is a logical ID, it isn't used by hardware. The address space
> >> concept in virtio-iommu allows to group endpoints together, so that they
> >> have the same address space. I thought it was pretty much the same as
> >> "domains" in VT-d? In any case, it is the same as domains in Linux. An
> >> IOASID provides a handle for communication between virtio-iommu
> device
> >> and
> >> driver, but unlike PASID, the IOASID number doesn't mean anything
> outside
> >> of virtio-iommu.
> >
> > Thanks. It's clear to me then.
> >
> > btw does it make more sense to use "domain id" instead of "IO address
> > space id"? For one, when talking about layered address spaces
> > usually parent address space is a superset of all child address spaces
> > which doesn't apply to this case, since either anonymous address
> > space or PASID-tagged address spaces are completely isolated. Instead>
> 'domain' is a more inclusive terminology to embrace multiple address
> > spaces. For two, 'domain' is better aligned to software terminology (e.g.
> > iommu_domain) is easier for people to catch up. :-)
> I do agree that the naming isn't great. I didn't use "domain" for various
> reasons (it also has a different meanings in ARM) but I keep regretting
> it. As there is no virtio-iommu code upstream yet, it is still possible to
> change this one.
> I find that "address space" was a good fit for the baseline device, but
> the name doesn't scale. When introducing PASIDs, the address space
> moves
> one level down in the programming model. It is contexts, anonymous or
> PASID-tagged, that should be called address spaces. I was considering
> replacing it with "domain", "container", "partition"...
> Even though I don't want to use too much Linux terminology (virtio isn't
> just Linux), "domain" is better fitted, somewhat neutral, and gets the
> point across. A domain has one or more input address spaces and a single
> output address space.
> When introducing nested translation to virtio-iommu (for the guest to have
> virtual machines itself), there will be one or more intermediate address
> spaces. Domains will be nested, with the terminology "parent domain" and
> "child domain". I only briefly looked at a programming model for this but
> I think we can nest virtio-iommus without much hassle.
> If there is no objection the next version will use "domain" in place of
> "address_space". The change is quite invasive at this point, but I believe
> that it will makes things more clear down the road.

Sounds good to me. Thanks.

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