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

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Message-ID: <20210922123114.GH327412@nvidia.com>
Date:   Wed, 22 Sep 2021 09:31:14 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     "Liu, Yi L" <yi.l.liu@...el.com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
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        "yi.l.liu@...ux.intel.com" <yi.l.liu@...ux.intel.com>,
        "Tian, Jun J" <jun.j.tian@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
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Subject: Re: [RFC 02/20] vfio: Add device class for /dev/vfio/devices

On Wed, Sep 22, 2021 at 01:07:11AM +0000, Tian, Kevin wrote:
> > 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? 

The "new state" is if the fops are pointing at the real fops or the
pre-fops, which in turn protects everything. You could imagine this as
some state in front of every fop call if you want.

> In this case what is the point of keeping device FD disabled even
> for the group path?

I have a feeling when you go through the APIs it will make sense to
have some symmetry here.

eg creating a device FD should have basically the same flow no matter
what triggers it, not confusing special cases where the group code
skips steps

Jason