On Mon, Jun 02, 2025, Alex Williamson wrote: > On Fri, 16 May 2025 16:07:26 -0700 > Sean Christopherson <sea...@google.com> 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. > > > > 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 > > Sorry for the delay.
Heh, no worries. ~2 weeks is downright prompt by my standards ;-) > Do you intend to take this through your trees? Yes, ideally, it would go into Paolo's kvm/next sooner than later (I'll start poking him if necessary). The s/token/eventfd rename creates an annoying conflict in kvm/x86.c with an in-flight patch (significant code movement between files). It would be nice to be able to rebase the in-flight patch instead of having to resolve a merge confict (the conflict itself isn't difficult to resolve, I just find it hard to visually review/audit the resolution due to the code movement).
