Re: [RFC PATCH 05/10] vfio: Create a vfio_device from vma lookup

From: Jason Gunthorpe <jgg@nvidia.com>
To: Alex Williamson <alex.williamson@redhat.com>
Cc: <cohuck@redhat.com>, <kvm@vger.kernel.org>,
	<linux-kernel@vger.kernel.org>, <peterx@redhat.com>
Subject: Re: [RFC PATCH 05/10] vfio: Create a vfio_device from vma lookup
Date: Thu, 25 Feb 2021 19:49:49 -0400	[thread overview]
Message-ID: <20210225234949.GV4247@nvidia.com> (raw)
In-Reply-To: <20210225152113.3e083b4a@omen.home.shazbot.org>

On Thu, Feb 25, 2021 at 03:21:13PM -0700, Alex Williamson wrote:

> This is where it gets tricky.  The vm_pgoff we get from
> file_operations.mmap is already essentially describing an offset from
> the base of a specific resource.  We could convert that from an absolute
> offset to a pfn offset, but it's only the bus driver code (ex.
> vfio-pci) that knows how to get the base, assuming there is a single
> base per region (we can't assume enough bits per region to store
> absolute pfn).  Also note that you're suggesting that all vfio mmaps
> would need to standardize on the vfio-pci implementation of region
> layouts.  Not that most drivers haven't copied vfio-pci, but we've
> specifically avoided exposing it as a fixed uAPI such that we could have
> the flexibility for a bus driver to implement regions offsets however
> they need.

Okay, well the bus driver owns the address space and the bus driver is
in control of the vm_pgoff. If it doesn't want to zap then it doesn't
need to do anything

vfio-pci can consistently use the index encoding and be fine

> So I'm not really sure what this looks like.  Within vfio-pci we could
> keep the index bits in place to allow unmmap_mapping_range() to
> selectively zap matching vm_pgoffs but expanding that to a vfio
> standard such that the IOMMU backend can also extract a pfn looks very
> limiting, or ugly.  Thanks,

Lets add a op to convert a vma into a PFN range. The map code will
pass the vma to the op and get back a pfn (or failure).

pci will switch the vm_pgoff to an index, find the bar base and
compute the pfn.

It is starting to look more and more like dma buf though

Jason