> From: Eric Auger <eric.auger@xxxxxxxxxx>
> Sent: Thursday, January 19, 2023 5:13 PM
>
> Hi Yi,
>
> On 1/17/23 14:49, Yi Liu wrote:
> > This is to avoid a circular refcount problem between the kvm struct and
> > the device file. KVM modules holds device/group file reference when the
> > device/group is added and releases it per removal or the last kvm reference
> > is released. This reference model is ok for the group since there is no
> > kvm reference in the group paths.
> >
> > But it is a problem for device file since the vfio devices may get kvm
> > reference in the device open path and put it in the device file release.
> > e.g. Intel kvmgt. This would result in a circular issue since the kvm
> > side won't put the device file reference if kvm reference is not 0, while
> > the vfio device side needs to put kvm reference in the release callback.
> >
> > To solve this problem for device file, let vfio provide release() which
> > would be called once kvm file is closed, it won't depend on the last kvm
> > reference. Hence avoid circular refcount problem.
> >
> > Suggested-by: Kevin Tian <kevin.tian@xxxxxxxxx>
> > Signed-off-by: Yi Liu <yi.l.liu@xxxxxxxxx>
> > ---
> > virt/kvm/vfio.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/virt/kvm/vfio.c b/virt/kvm/vfio.c
> > index 0f54b9d308d7..525efe37ab6d 100644
> > --- a/virt/kvm/vfio.c
> > +++ b/virt/kvm/vfio.c
> > @@ -364,7 +364,7 @@ static int kvm_vfio_create(struct kvm_device *dev,
> u32 type);
> > static struct kvm_device_ops kvm_vfio_ops = {
> > .name = "kvm-vfio",
> > .create = kvm_vfio_create,
> > - .destroy = kvm_vfio_destroy,
> Is it safe to simply remove the destroy cb as it is called from
> kvm_destroy_vm/kvm_destroy_devices?
>
According to the definition .release is considered as an alternative
method to free the device:
/*
* Destroy is responsible for freeing dev.
*
* Destroy may be called before or after destructors are called
* on emulated I/O regions, depending on whether a reference is
* held by a vcpu or other kvm component that gets destroyed
* after the emulated I/O.
*/
void (*destroy)(struct kvm_device *dev);
/*
* Release is an alternative method to free the device. It is
* called when the device file descriptor is closed. Once
* release is called, the destroy method will not be called
* anymore as the device is removed from the device list of
* the VM. kvm->lock is held.
*/
void (*release)(struct kvm_device *dev);
Did you see any specific problem of moving this stuff to release?
