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. To address the use case where huge numbers of VMs are being created without _any_ possibility for irqbypass, KVM should probably add a KVM_IRQFD_FLAG_NO_IRQBYPASS flag so that userspace can opt-out on a per-IRQ basis. I already proposed a KVM module param[2] to let userspace disable IRQ bypass, but that obviously affects all IRQs in all VMs. It might suffice for most use cases, but I can imagine scenarios where the VMM wants to be more selective, e.g. when it *knows* a KVM_IRQFD isn't eligible for bypass. And both of those require userspace changes. 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. But that's a future problem. [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 (7): 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 drivers/vfio/pci/vfio_pci_intrs.c | 5 +- drivers/vhost/vdpa.c | 4 +- include/linux/irqbypass.h | 38 +++--- virt/kvm/eventfd.c | 3 +- virt/lib/irqbypass.c | 185 ++++++++++-------------------- 5 files changed, 88 insertions(+), 147 deletions(-) base-commit: 782f9feaa9517caf33186dcdd6b50a8f770ed29b -- 2.49.0.504.g3bcea36a83-goog
