RE: [PATCH v2 1/2] vfio/type1: Simplify bus_type determination

From: "Tian, Kevin" <kevin.tian@intel.com>
To: Alex Williamson <alex.williamson@redhat.com>,
	Robin Murphy <robin.murphy@arm.com>
Cc: "cohuck@redhat.com" <cohuck@redhat.com>,
	"jgg@nvidia.com" <jgg@nvidia.com>,
	"iommu@lists.linux.dev" <iommu@lists.linux.dev>,
	"iommu@lists.linux-foundation.org"
	<iommu@lists.linux-foundation.org>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH v2 1/2] vfio/type1: Simplify bus_type determination
Date: Thu, 23 Jun 2022 08:46:45 +0000	[thread overview]
Message-ID: <BN9PR11MB5276A79834CCB5954A3025DF8CB59@BN9PR11MB5276.namprd11.prod.outlook.com> (raw)
In-Reply-To: <20220622161721.469fc9eb.alex.williamson@redhat.com>

> From: Alex Williamson <alex.williamson@redhat.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.