The two primary goals of this series are to make the irqbypass concept easier to understand, and to address the terrible performance that can result from using a list to track connections.
For the first goal, track the producer/consumer "tokens" as eventfd context pointers instead of opaque "void *". Supporting arbitrary token types was dead infrastructure when it was added 10 years ago, and nothing has changed since. Taking an opaque token makes a very simple concept (device signals eventfd; KVM listens to eventfd) unnecessarily difficult to understand. Burying that simple behind a layer of obfuscation also makes the overall code more brittle, as callers can pass in literally anything. I.e. passing in a token that will never be paired would go unnoticed. For the performance issue, use an xarray. I'm definitely not wedded to an xarray, but IMO it doesn't add meaningful complexity (even requires less code), and pretty much Just Works. Like tried this a while back[1], but the implementation had undesirable behavior changes and stalled out. Note, I want to do more aggressive cleanups of irqbypass at some point, e.g. not reporting an error to userspace if connect() fails is awful behavior for environments that want/need irqbypass to always work. And KVM shold probably have a KVM_IRQFD_FLAG_NO_IRQBYPASS if a VM is never going to use device posted interrupts. But those are future problems. v2: - Collect reviews. [Kevin, Michael] - Track the pointer as "struct eventfd_ctx *eventfd" instead of "void *token". [Alex] - Fix typos and stale comments. [Kevin, Binbin] - Use "trigger" instead of the null token/eventfd pointer on failure in vfio_msi_set_vector_signal(). [Kevin] - Drop a redundant "tmp == consumer" check from patch 3. [Kevin] - Require producers to pass in the line IRQ number. v1: https://lore.kernel.org/all/20250404211449.1443336-1-sea...@google.com [1] https://lore.kernel.org/all/20230801115646.33990-1-lik...@tencent.com [2] https://lore.kernel.org/all/20250401161804.842968-1-sea...@google.com Sean Christopherson (8): irqbypass: Drop pointless and misleading THIS_MODULE get/put irqbypass: Drop superfluous might_sleep() annotations irqbypass: Take ownership of producer/consumer token tracking irqbypass: Explicitly track producer and consumer bindings irqbypass: Use paired consumer/producer to disconnect during unregister irqbypass: Use guard(mutex) in lieu of manual lock+unlock irqbypass: Use xarray to track producers and consumers irqbypass: Require producers to pass in Linux IRQ number during registration arch/x86/kvm/x86.c | 4 +- drivers/vfio/pci/vfio_pci_intrs.c | 10 +- drivers/vhost/vdpa.c | 10 +- include/linux/irqbypass.h | 46 ++++---- virt/kvm/eventfd.c | 7 +- virt/lib/irqbypass.c | 190 +++++++++++------------------- 6 files changed, 107 insertions(+), 160 deletions(-) base-commit: 7ef51a41466bc846ad794d505e2e34ff97157f7f -- 2.49.0.1112.g889b7c5bd8-goog
