Re: [PATCH v3 4/5] KVM/vfio: Use file-based reference counting for KVM

[Jason J. Herne]

On 9/24/26 4:58 AM, Steffen Eiden wrote:
> Replace manual module reference counting with file-based reference
> counting for KVM integration. Previously, VFIO used symbol_get() to
> obtain function pointers for kvm_get_kvm_safe() and kvm_put_kvm(),
> then manually tracked module references through these symbols. This
> approach required storing the put_kvm function pointer in each device
> and carefully managing symbol references.
>
> Pass struct file pointers instead of struct kvm pointers throughout the
> VFIO-KVM interface, leveraging the kernel's existing file reference
> counting via get_file()/get_file_active() and fput(). Convert the x86
> page-track API and update s390 vfio to use file_to_kvm_<arch>(). This
> simplifies the code and removes all remaining externally exported
> symbols for KVM which would appear twice for when a second concurrent
> KVM module is introduced.
>
> Suggested-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Co-developed-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Steffen Eiden <seiden@xxxxxxxxxxxxx>
> ---
>   arch/s390/include/asm/kvm_host_s390.h |  2 +-
>   arch/s390/kvm/s390/pci.c              |  9 ++++--
>   arch/x86/include/asm/kvm_page_track.h |  8 ++---
>   arch/x86/kvm/mmu/page_track.c         | 22 +++++++++-----
>   drivers/s390/crypto/vfio_ap_ops.c     | 20 ++++++++----
>   drivers/vfio/group.c                  | 11 ++++++-
>   drivers/vfio/vfio.h                   | 12 ++++----
>   drivers/vfio/vfio_main.c              | 57 +++++++++++------------------------
>   include/linux/vfio.h                  |  5 ++-
>   virt/kvm/vfio.c                       | 13 +++++---
>   10 files changed, 85 insertions(+), 74 deletions(-)
> ...
> diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
> index 940c0ff668be..556e643244f2 100644
> --- a/drivers/s390/crypto/vfio_ap_ops.c
> +++ b/drivers/s390/crypto/vfio_ap_ops.c
> @@ -1822,17 +1822,27 @@ static const struct attribute_group *vfio_ap_mdev_attr_groups[] = {
>   
>   /**
>    * vfio_ap_mdev_set_kvm - sets all data for @matrix_mdev that are needed
> - * to manage AP resources for the guest whose state is represented by @kvm
> + * to manage AP resources for the guest whose state is represented by
> + * @kvm_file
>    *
>    * @matrix_mdev: a mediated matrix device
> - * @kvm: reference to KVM instance
> + * @kvm_file: the KVM VM file this vfio device is associated with
>    *
> - * Return: 0 if no other mediated matrix device has a reference to @kvm;
> + * Return: 0 if no other mediated matrix device has a reference to the VM;
>    * otherwise, returns an -EPERM.
>    */
>   static int vfio_ap_mdev_set_kvm(struct ap_matrix_mdev *matrix_mdev,
> -				struct kvm *kvm)
> +				struct file *kvm_file)
>   {
> +	struct kvm *kvm;
> +
> +	if (!kvm_file)
> +		return -ENOENT;
> +
> +	kvm = file_to_kvm_s390(kvm_file);
> +	if (!kvm)
> +		return -ENOENT;
> +
>   	if (kvm->arch.crypto.crycbd) {
>   		get_update_locks_for_kvm(kvm);
>   		if (kvm->arch.crypto.pqap_hook) {
> @@ -1841,7 +1851,6 @@ static int vfio_ap_mdev_set_kvm(struct ap_matrix_mdev *matrix_mdev,
>   		}
>   		kvm->arch.crypto.pqap_hook = &matrix_mdev->pqap_hook;
>   
> -		kvm_get_kvm(kvm);
>   		matrix_mdev->kvm = kvm;
>   		vfio_ap_mdev_update_guest_apcb(matrix_mdev);
>   		release_update_locks_for_kvm(kvm);
> @@ -1894,7 +1903,6 @@ static void vfio_ap_mdev_unset_kvm(struct ap_matrix_mdev *matrix_mdev)
>   		matrix_mdev->kvm = NULL;
>   
>   		release_update_locks_for_kvm(kvm);
> -		kvm_put_kvm(kvm);
>   	}
>   }
>   
Reviewed-by: Jason J. Herne <jjherne@xxxxxxxxxxxxx>