On Fri, Apr 04, 2025 at 02:14:42PM -0700, Sean Christopherson wrote:
> 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-likexu@tencent.com
> [2] https://lore.kernel.org/all/20250401161804.842968-1-seanjc@google.com
vdpa changes seem minor, so
Acked-by: Michael S. Tsirkin <mst@redhat.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