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Date:   Mon, 16 May 2022 23:21:19 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     Matthew Rosato <mjrosato@linux.ibm.com>,
        linux-s390@vger.kernel.org, alex.williamson@redhat.com,
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Subject: Re: [PATCH v7 17/22] vfio-pci/zdev: add open/close device hooks
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 <20220513191509.272897-18-mjrosato@linux.ibm.com>
 <20220516172734.GE1343366@nvidia.com>
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On Mon, May 16, 2022 at 02:27:34PM -0300, Jason Gunthorpe wrote:
> Normally you'd want to do what is kvm_s390_pci_register_kvm() here,
> where a failure can be propogated but then you have a race condition
> with the kvm.
> 
> Blech, maybe it is time to just fix this race condition permanently,
> what do you think? (I didn't even compile it)

This is roughly were I was planning to get to, with one difference:
I don't think we need or even want the VFIO_DEVICE_NEEDS_KVM flag.
Instead just propagation ->kvm to the device whenever it is set and
let drivers that have a hard requirements on it like gvt fail if it
isn't there.  This could still allow using vfio for userspace PCI
drivers on s390 for example or in general allow expressing a soft
requirement, just without the whole notifier mess.

The other question is if we even need an extra reference per device,
can't we hold the group reference until all devices are gone
anyway?  That would remove the need to include kvm_host.h in the
vfio code.