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Date:   Wed, 16 Sep 2020 11:44:59 -0300
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     "Tian, Kevin" <kevin.tian@intel.com>
CC:     Alex Williamson <alex.williamson@redhat.com>,
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Subject: Re: [PATCH v7 00/16] vfio: expose virtual Shared Virtual Addressing
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On Wed, Sep 16, 2020 at 01:19:18AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@nvidia.com>
> > Sent: Tuesday, September 15, 2020 10:29 PM
> >
> > > Do they need a device at all?  It's not clear to me why RID based
> > > IOMMU management fits within vfio's scope, but PASID based does not.
> > 
> > In RID mode vfio-pci completely owns the PCI function, so it is more
> > natural that VFIO, as the sole device owner, would own the DMA mapping
> > machinery. Further, the RID IOMMU mode is rarely used outside of VFIO
> > so there is not much reason to try and disaggregate the API.
> 
> It is also used by vDPA.

A driver in VDPA, not VDPA itself.

> > PASID on the other hand, is shared. vfio-mdev drivers will share the
> > device with other kernel drivers. PASID and DMA will be concurrent
> > with VFIO and other kernel drivers/etc.
> 
> Looks you are equating PASID to host-side sharing, while ignoring 
> another valid usage that a PASID-capable device is passed through
> to the guest through vfio-pci and then PASID is used by the guest
> for guest-side sharing. In such case, it is an exclusive usage in host
> side and then what is the problem for VFIO to manage PASID given
> that vfio-pci completely owns the function?

This is no different than vfio-pci being yet another client to
/dev/sva

Jason