[PATCH v4 0/3] KVM: s390: Introducing kvm_arch_set_irq_inatomic Fast Inject

Douglas Freimuth
2026-04-23T23:53:31Z

S390 needs this series of three patches in order to enable a non-blocking
path for irqfd injection on s390 via kvm_arch_set_irq_inatomic(). Before
these changes, kvm_arch_set_irq_inatomic() would just return -EWOULDBLOCK
and place all interrupts on the global work queue, which must subsequently
be processed by a different thread. This series of patches implements an
s390 version of inatomic and is relevant to virtio-blk and virtio-net and
was tested against virtio-pci and virtio-ccw.

The inatomic fast path cannot lose control since it is running with
interrupts disabled. This meant making the following changes that exist on
the slow path today. First, the adapter_indicators page needs to be mapped
since it is accessed with interrupts disabled, so we added map/unmap
functions. Second, access to shared resources between the fast and slow
paths needed to be changed from mutex and semaphores to raw_spin_lock's.
Finally, the memory allocation on the slow path utilizes GFP_KERNEL_ACCOUNT
but we had to implement the fast path with GFP_ATOMIC allocation. Each of
these enhancements were required to prevent blocking on the fast inject
path.

Statistical counters have been added to enable analysis of irq injection on
the fast path and slow path including io_390_inatomic, io_flic_inject_airq,
io_set_adapter_int and io_390_inatomic_adapter_masked. And counters have
been added to analyze map/unmap of the adapter_indicator
pages in non-Secure Execution environments and to track fencing of Fast
Inject in Secure Execution environments. In order to take advantage of this
kernel series with virtio-pci, a QEMU that includes the
's390x/pci: set kvm_msi_via_irqfd_allowed' fix is needed. Additionally,
the guest xml needs a thread pool and threads explicitly assigned per disk
device using the common way of defining threads for disks.

Patch 1 enables map/unmap of adapter indicator pages but for Secure
Execution environments it avoids the long term mapping.

v3->v4: In kvm_s390_adapter_unmap mark_page_dirty using guest address.
v3->v4: In kvm_s390_unmap_all_adapters_pv mark_page_dirty using guest addr.
v3->v4: In kvm_s390_unmap_all_adapters_pv decrement nr_maps.
v3->v4: In kvm_s390_adapter_unmap hold maps->lock through list deletion.
v3->v4: In kvm_s390_destroy_adapters hold the maps->lock until list empty.
v3->v4: In kvm_s390_unmap_all_adapters_pv hold maps->lock to empty list.
v3->v4: Change maps_lock and ais_lock to raw_spinlock for RT kernel case.

Douglas Freimuth (3):
KVM: s390: Add map/unmap ioctl and clean mappings post-guest
KVM: s390: Enable adapter_indicators_set to use mapped pages
KVM: s390: Introducing kvm_arch_set_irq_inatomic fast inject

arch/s390/include/asm/kvm_host.h | 11 +-
arch/s390/kvm/interrupt.c | 401 ++++++++++++++++++++++++++-----
arch/s390/kvm/kvm-s390.c | 56 ++++-
arch/s390/kvm/kvm-s390.h | 3 +-
4 files changed, 402 insertions(+), 69 deletions(-)

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2.52.0