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

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Date:   Thu, 23 Jun 2022 14:35:52 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Robin Murphy <robin.murphy@....com>,
        "cohuck@...hat.com" <cohuck@...hat.com>,
        "jgg@...dia.com" <jgg@...dia.com>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] vfio/type1: Simplify bus_type determination

On Thu, 23 Jun 2022 08:46:45 +0000
"Tian, Kevin" <kevin.tian@...el.com> wrote:

> > From: Alex Williamson <alex.williamson@...hat.com>
> > Sent: Thursday, June 23, 2022 6:17 AM
> >   
> > >
> > >  	ret = -EIO;
> > > -	domain->domain = iommu_domain_alloc(bus);
> > > +	domain->domain = iommu_domain_alloc(iommu_api_dev->dev-
> > >bus);  
> > 
> > It makes sense to move away from a bus centric interface to iommu ops
> > and I can see that having a device interface when we have device level
> > address-ability within a group makes sense, but does it make sense to
> > only have that device level interface?  For example, if an iommu_group
> > is going to remain an aspect of the iommu subsystem, shouldn't we be
> > able to allocate a domain and test capabilities based on the group and
> > the iommu driver should have enough embedded information reachable
> > from
> > the struct iommu_group to do those things?  This "perform group level
> > operations based on an arbitrary device in the group" is pretty klunky.
> > Thanks,
> >   
> 
> This sounds a right thing to do.
> 
> btw another alternative which I'm thinking of is whether vfio_group
> can record the bus info when the first device is added to it in
> __vfio_register_dev(). Then we don't need a group interface from
> iommu to test if vfio is the only user having such requirement.

That might be more simple, but it's just another variation on vfio
picking an arbitrary device from a group to satisfy the iommu interface
rather than operating on an iommu subsystem provided object.  Thanks,

Alex