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

["Tian, Kevin" <kevin.tian@intel.com>]

> From: Jean-Philippe Brucker [mailto:jean-philippe.brucker@arm.com]
> Sent: Monday, April 24, 2017 11:06 PM
> >>>>   1. Attach device
> >>>>   ----------------
> >>>>
> >>>> struct virtio_iommu_req_attach {
> >>>> 	le32	address_space;
> >>>> 	le32	device;
> >>>> 	le32	flags/reserved;
> >>>> };
> >>>>
> >>>> Attach a device to an address space. 'address_space' is an identifier
> >>>> unique to the guest. If the address space doesn't exist in the IOMMU
> >>>
> >>> Based on your description this address space ID is per operation right?
> >>> MAP/UNMAP and page-table sharing should have different ID spaces...
> >>
> >> I think it's simpler if we keep a single IOASID space per virtio-iommu
> >> device, because the maximum number of address spaces (described by
> >> ioasid_bits) might be a restriction of the pIOMMU. For page-table
> sharing
> >> you still need to define which devices will share a page directory using
> >> ATTACH requests, though that interface is not set in stone.
> >
> > got you. yes VM is supposed to consume less IOASIDs than physically
> > available. It doesn’t hurt to have one IOASID space for both IOVA
> > map/unmap usages (one IOASID per device) and SVM usages (multiple
> > IOASIDs per device). The former is digested by software and the latter
> > will be bound to hardware.
> >
> 
> Hmm, I'm using address space indexed by IOASID for "classic" IOMMU, and
> then contexts indexed by PASID when talking about SVM. So in my mind an
> address space can have multiple sub-address-spaces (contexts). Number of
> IOASIDs is a limitation of the pIOMMU, and number of PASIDs is a
> limitation of the device. Therefore attaching devices to address spaces
> would update the number of available contexts in that address space. The
> terminology is not ideal, and I'd be happy to change it for something more
> clear.
> 

(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)?

Thanks
Kevin
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