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

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Date:   Thu, 23 Jun 2022 22:50:30 -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 Thu, Jun 23, 2022 at 05:00:44PM -0600, Alex Williamson wrote:

> > >> +struct vfio_device *vfio_device_get_from_iommu(struct iommu_group *iommu_group)
> > >> +{
> > >> +	struct vfio_group *group = vfio_group_get_from_iommu(iommu_group);
> > >> +	struct vfio_device *device;  
> > > 
> > > Check group for NULL.  
> > 
> > OK - FWIW in context this should only ever make sense to call with an 
> > iommu_group which has already been derived from a vfio_group, and I did 
> > initially consider a check with a WARN_ON(), but then decided that the 
> > unguarded dereference would be a sufficiently strong message. No problem 
> > with bringing that back to make it more defensive if that's what you prefer.
> 
> A while down the road, that's a bit too much implicit knowledge of the
> intent and single purpose of this function just to simply avoid a test.

I think we should just pass the 'struct vfio_group *' into the
attach_group op and have this API take that type in and forget the
vfio_group_get_from_iommu().

At this point there is little justification for
vfio_group_get_from_iommu() existing at all, it should be folded into
the one use in vfio_group_find_or_alloc() and the locking widened so
we don't have the unlock/alloc/lock race that requires it to be called
twice.

> I'd lean towards Kevin's idea that we could store bus_type on the
> vfio_group and pass that to type1, with the same assumptions we're
> making in the commit log that it's consistent, but that doesn't get us
> closer to the long term plan of dropping the bus_type interfaces
> AIUI.

Right, the point is to get a representative struct device here to use.

Jason