Re: [PATCH 1/1] vfio: remove VFIO_GROUP_NOTIFY_SET_KVM

[Jason Gunthorpe <jgg@nvidia.com>]

On Wed, May 18, 2022 at 11:33:37AM -0400, Matthew Rosato wrote:
> On 5/18/22 11:12 AM, Jason Gunthorpe wrote:
> > On Wed, May 18, 2022 at 10:37:48AM -0400, Matthew Rosato wrote:
> > > 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
> > > > +++ 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...
> > 
> > Ugh, my other plan was to have the driver itself capture the kvm, ie
> > we lock the group_rwsem to keep the group->kvm valid and then pass the
> > kvm to open_device in some way, then the driver can kvm_get_kvm() it
> > 
> 
> Hrm... If we did that we would have to re-evaluate some other usage of the
> rwsem e.g. if driver open_device calls vfio_register_iommu_notifier it will
> try to get the rwsem but it's already locked.

Ugh, yes, it means removing the other notifier callback too, which I
was expecting to do as well

Maybe we could split the lock for just this patch though.

> > Alternatively, I don't know why kvm_get_kvm() is an exported symbol
> > when it is just calling refcount_inc() - inlining it would be an
> > improvement I think.
> 
> I think that would work for kvm_get_kvm, but kvm_put_kvm (which we also
> need) calls kvm_destroy_kvm after the refcount_dec and that can't be inlined

Indeed.

Jason