Merge tag 'kvm-s390-next-7.4-1' of git://git.kernel.org/pub/scm/linux/kernel/git/kvms390/linux into HEAD

KVM/vfio: Use file-based reference counting for KVM

This series switches the KVM-VFIO interface and external consumers over to
standard file-based reference counting, eliminating all external KVM symbol
exports.

Currently, VFIO integrates with KVM by looking up kvm_get_kvm_safe() and
kvm_put_kvm() dynamically using symbol_get(), manually tracking module
reference counts and storing a put_kvm function pointer in struct
vfio_device.

In the ARM64-on-s390 architecture, a second concurrent KVM module
(kvm-arm64) is introduced alongside native KVM to host hardware-accelerated
ARM64 guests. Having exported global symbols (like kvm_get_kvm/kvm_put_kvm)
creates symbol conflicts and prevents clean coexistence of two KVM modules.

Additionally, the file based counting simplifies the code and reuses
the existing fs refcounting.

Instead of passing raw KVM pointers and managing module symbols manually, the
interface now passes the underlying VM file descriptor throughout VFIO and
associated architecture subsystems. To safely extract the KVM instance from a
file, an architecture-namespaced helper mechanism is introduced that verifies
the file belongs to the expected KVM implementation before accessing its
internal state. A back-pointer from the KVM instance to its associated file is
maintained across its lifecycle so subsystems can safely acquire file
references on demand. Finally, with VFIO, architecture page tracking, and
device hooks converted to use file references, the remaining KVM reference-
counting exports are restricted strictly to internal KVM modules.

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