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

[Paolo Bonzini]

On Fri, Oct 9, 2026 at 4:28 PM Christian Borntraeger
<borntraeger@xxxxxxxxxxxxx> wrote:
>
>
> Paolo,
>
> The following changes since commit df2908090cda368b01ff43709f51890076c56157:
>
>   Linux 7.3-rc2 (2026-09-06 15:07:20 -0700)
>
> are available in the Git repository at:
>
>   git://git.kernel.org/pub/scm/linux/kernel/git/kvms390/linux.git  tags/kvm-s390-next-7.4-1
>
> for you to fetch changes up to c22295d0d75f5590fdde5493cb90e2d50d5f508e:
>
>   KVM: Remove unused file_is_kvm (2026-10-02 10:40:01 +0200)

Pulled, thanks.

Paolo

>
> The tag points to the vfio_file_reference branch of the kvms390 repo
> https://git.kernel.org/pub/scm/linux/kernel/git/kvms390/linux.git/log/?h=vfio_file_reference
>
>
>
> ----------------------------------------------------------------
> 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.
>
> ----------------------------------------------------------------
> Steffen Eiden (6):
>       KVM: Introduce file_to_kvm_<arch>() infrastructure
>       KVM: Add file back-pointer to struct kvm
>       KVM: x86: Use file_to_kvm_x86() in SEV
>       KVM/vfio: Use file-based reference counting for KVM
>       KVM: Restrict kvm_get_kvm/kvm_put_kvm export to internal KVM modules
>       KVM: Remove unused file_is_kvm
>
>  arch/s390/include/asm/kvm_host_s390.h |  4 ++-
>  arch/s390/kvm/s390/pci.c              |  9 ++++--
>  arch/x86/include/asm/kvm_host.h       |  2 ++
>  arch/x86/include/asm/kvm_page_track.h | 10 +++---
>  arch/x86/kvm/Makefile                 |  4 +--
>  arch/x86/kvm/mmu/page_track.c         | 22 +++++++++-----
>  arch/x86/kvm/svm/sev.c                |  8 ++---
>  drivers/s390/crypto/vfio_ap_ops.c     | 20 ++++++++----
>  drivers/vfio/device_cdev.c            |  2 +-
>  drivers/vfio/group.c                  | 13 ++++++--
>  drivers/vfio/vfio.h                   | 14 ++++-----
>  drivers/vfio/vfio_main.c              | 57 +++++++++++------------------------
>  include/linux/kvm_host.h              | 19 +++++++++++-
>  include/linux/vfio.h                  |  5 ++-
>  virt/kvm/kvm_main.c                   | 21 +++++++++----
>  virt/kvm/vfio.c                       | 13 +++++---
>  16 files changed, 132 insertions(+), 91 deletions(-)
>