linux-kernel - RE: [RFC 02/20] vfio: Add device class for /dev/vfio/devices

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Message-ID: <BN9PR11MB543322943B84EFAC88AD25438CA29@BN9PR11MB5433.namprd11.prod.outlook.com>
Date:   Wed, 22 Sep 2021 01:07:11 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     "Liu, Yi L" <yi.l.liu@...el.com>,
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Subject: RE: [RFC 02/20] vfio: Add device class for /dev/vfio/devices

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Wednesday, September 22, 2021 8:55 AM
> 
> On Tue, Sep 21, 2021 at 11:56:06PM +0000, Tian, Kevin wrote:
> > > The opened atomic is aweful. A newly created fd should start in a
> > > state where it has a disabled fops
> > >
> > > The only thing the disabled fops can do is register the device to the
> > > iommu fd. When successfully registered the device gets the normal fops.
> > >
> > > The registration steps should be done under a normal lock inside the
> > > vfio_device. If a vfio_device is already registered then further
> > > registration should fail.
> > >
> > > Getting the device fd via the group fd triggers the same sequence as
> > > above.
> > >
> >
> > Above works if the group interface is also connected to iommufd, i.e.
> > making vfio type1 as a shim. In this case we can use the registration
> > status as the exclusive switch. But if we keep vfio type1 separate as
> > today, then a new atomic is still necessary. This all depends on how
> > we want to deal with vfio type1 and iommufd, and possibly what's
> > discussed here just adds another pound to the shim option...
> 
> No, it works the same either way, the group FD path is identical to
> the normal FD path, it just triggers some of the state transitions
> automatically internally instead of requiring external ioctls.
> 
> The device FDs starts disabled, an internal API binds it to the iommu
> via open coding with the group API, and then the rest of the APIs can
> be enabled. Same as today.
> 

Still a bit confused. if vfio type1 also connects to iommufd, whether 
the device is registered can be centrally checked based on whether
an iommu_ctx is recorded. But if type1 doesn't talk to iommufd at
all, don't we still need introduce a new state (calling it 'opened' or
'registered') to protect the two interfaces? In this case what is the
point of keeping device FD disabled even for the group path?