Re: [PATCH 07/14] vfio: simplify iommu group allocation for mediated devices

From: Jason Gunthorpe <jgg@ziepe.ca>
To: Christoph Hellwig <hch@lst.de>
Cc: Alex Williamson <alex.williamson@redhat.com>,
	Diana Craciun <diana.craciun@oss.nxp.com>,
	Cornelia Huck <cohuck@redhat.com>,
	Kirti Wankhede <kwankhede@nvidia.com>,
	Eric Auger <eric.auger@redhat.com>,
	kvm@vger.kernel.org
Subject: Re: [PATCH 07/14] vfio: simplify iommu group allocation for mediated devices
Date: Wed, 25 Aug 2021 09:21:44 -0300	[thread overview]
Message-ID: <20210825122144.GV543798@ziepe.ca> (raw)
In-Reply-To: <20210825053237.GB26806@lst.de>

On Wed, Aug 25, 2021 at 07:32:37AM +0200, Christoph Hellwig wrote:
> On Tue, Aug 24, 2021 at 09:19:16PM -0300, Jason Gunthorpe wrote:
> > The mechanism looks fine, but I think the core code is much clearer if
> > the name is not 'mediated' but 'sw_iommu' or something that implies
> > the group is running with a software page table. mediated has become
> > so overloaded in this code.
> 
> I thought that was sort of the definition of mediated - there needs to
> ben entify that "mediates" access so that a user of this interface
> can't trigger undmediated DMA to arbitrary addresses.  My other choice
> that I used for a while was "virtual".  sw_iommu sounds a little clumsy.

vfio mediated is really some toolbox of different features that a
driver can use to build what it wants.

For instance Intel is looking at this concept of a mediated device
that uses PASID for the IOMMU. It would have a real IOMMU, use real
IOMMU page tables and in this language it would create some PASID
group, not a "sw_iommu" group.

This feature is about creating a device that is not connected to a HW
IO page table (at least by the VFIO iommu code) but the IO page table
is held in software and accessed by the VFIO driver through the pin
API.

virtual_iommu is somewhat overloaded with the idea of a vIOMMU created
by qemu and stuffed into a guest..

"domainless" might work but I also find it confusing that the iommu
code uses the word domain to refer to a HW IO page table :\

Maybe "sw io page table" ?

Jason