Re: [PATCH v4 0/6] KVM/vfio: Use file-based reference counting for KVM

[Janosch Frank]

On 9/28/26 3:07 PM, Steffen Eiden wrote:
> This series switches the KVM-VFIO interface and external consumers over to
> standard file-based reference counting, eliminating all external KVM symbol
> exports.
>
> This is a spin-off for the arm on s390 series (old version: [1]) to
> address all the relevant persons w/o polluting their mailbox. The
> arm-on-s390 series now dropped those patches and depends on this series
> (for beeing able to link with KVM=y)
>
> 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.
>
> 	Steffen
Series
Acked-by: Janosch Frank <frankja@xxxxxxxxxxxxx>