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. Do you intend to take this through your trees? Reviewed-by: Alex Williamson <alex.william...@redhat.com>
