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-lik...@tencent.com > [2] https://lore.kernel.org/all/20250401161804.842968-1-sea...@google.com
vdpa changes seem minor, so Acked-by: Michael S. Tsirkin <m...@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
