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Date:   Wed, 18 May 2022 10:37:48 -0400
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Subject: Re: [PATCH 1/1] vfio: remove VFIO_GROUP_NOTIFY_SET_KVM
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From:   Matthew Rosato <mjrosato@linux.ibm.com>
To:     jgg@nvidia.com, alex.williamson@redhat.com
Cc:     cohuck@redhat.com, borntraeger@linux.ibm.com,
        jjherne@linux.ibm.com, akrowiak@linux.ibm.com, pasic@linux.ibm.com,
        zhenyuw@linux.intel.com, zhi.a.wang@intel.com, hch@infradead.org,
        intel-gfx@lists.freedesktop.org,
        intel-gvt-dev@lists.freedesktop.org, linux-s390@vger.kernel.org,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20220517180851.166538-1-mjrosato@linux.ibm.com>
 <20220517180851.166538-2-mjrosato@linux.ibm.com>
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On 5/17/22 2:08 PM, Matthew Rosato wrote:
> Rather than relying on a notifier for associating the KVM with
> the group, let's assume that the association has already been
> made prior to device_open.  The first time a device is opened
> associate the group KVM with the device.
> 
> Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
> Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>

...

> diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
> index cfcff7764403..c5d421eda275 100644
> --- a/drivers/vfio/vfio.c
> +++ b/drivers/vfio/vfio.c
> @@ -10,6 +10,7 @@
>    * Author: Tom Lyon, pugs@cisco.com
>    */
>   
> +#include "linux/kvm_host.h"
>   #include <linux/cdev.h>
>   #include <linux/compat.h>
>   #include <linux/device.h>
> @@ -1083,6 +1084,13 @@ static struct file *vfio_device_open(struct vfio_device *device)
>   
>   	mutex_lock(&device->dev_set->lock);
>   	device->open_count++;
> +	down_write(&device->group->group_rwsem);
> +	if (device->open_count == 1 && device->group->kvm) {
> +		device->kvm = device->group->kvm;
> +		kvm_get_kvm(device->kvm);

Did some more compile testing, since vfio has no hard kvm dependency, 
kvm_get_kvm and kvm_put_kvm are an issue if KVM is a module while vfio 
is built-in...