Re: [PATCH v10 0/3] KVM: s390: Introducing kvm_arch_set_irq_inatomic Fast Inject

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

On Thu,  4 Jun 2026 21:27:52 +0200
Douglas Freimuth <freimuth@linux.ibm.com> wrote:

> 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 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.
> 
> s390 doesn't support a PREEMPT_RT kernel and this patch doesn't either.
> Given this fact, we are not using raw_spin_lock instead we are using
> regular spin_lock.
> 
> 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_no_inject. 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.
> 

Whole series:
Acked-by: Claudio Imbrenda <imbrenda@linux.ibm.com>


> v9->v10: Streamline logic in register_io_adapter()
> 
> 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/intercept.c        |   5 +-
>  arch/s390/kvm/interrupt.c        | 559 ++++++++++++++++++++++++-------
>  arch/s390/kvm/kvm-s390.c         |  30 +-
>  arch/s390/kvm/kvm-s390.h         |   5 +-
>  5 files changed, 488 insertions(+), 122 deletions(-)
>