linux-kernel - Re: [PATCH v2 1/2] vfio/type1: Simplify bus

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Date:   Fri, 24 Jun 2022 11:56:27 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     Robin Murphy <robin.murphy@....com>, cohuck@...hat.com,
        iommu@...ts.linux.dev, iommu@...ts.linux-foundation.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] vfio/type1: Simplify bus_type determination

On Fri, Jun 24, 2022 at 08:28:31AM -0600, Alex Williamson wrote:

> > > That's essentially what I'm suggesting, the vfio_group is passed as an
> > > opaque pointer which type1 can use for a
> > > vfio_group_for_each_vfio_device() type call.  Thanks,  
> > 
> > I don't want to add a whole vfio_group_for_each_vfio_device()
> > machinery that isn't actually needed by anything.. This is all
> > internal, we don't need to design more than exactly what is needed.
> > 
> > At this point if we change the signature of the attach then we may as
> > well just pass in the representative vfio_device, that is probably
> > less LOC overall.
> 
> That means that vfio core still needs to pick an arbitrary
> representative device, which I find in fundamental conflict to the
> nature of groups.

Well, this is where iommu is going, I think Robin has explained this
view well enough.

Ideally we'd move VFIO away from trying to attach groups and attach
when the device FD is opened, I view this as a micro step in that
direction.

> Type1 is the interface to the IOMMU API, if through the IOMMU API we
> can make an assumption that all devices within the group are
> equivalent for a given operation, that should be done in type1 code,
> not in vfio core.

iommu_group is part of the core code, if the representative device
assumption stems from the iommu_group then the core code can safely
make it.

> A for-each interface is commonplace and not significantly more code
> or design than already proposed.

Except that someone else might get the idea to use it for something
completely inappropriate.

Jason