Re: [PATCH v2 0/8] irqbypass: Cleanups and a perf improvement

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, May 16, 2025 at 04:07:26PM -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.
> 
> 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.


VDPA bits:

Acked-by: Michael S. Tsirkin <mst@redhat.com>

> v1: https://lore.kernel.org/all/20250404211449.1443336-1-seanjc@google.com
> 
> [1] https://lore.kernel.org/all/20230801115646.33990-1-likexu@tencent.com
> [2] https://lore.kernel.org/all/20250401161804.842968-1-seanjc@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
> -- 
> 2.49.0.1112.g889b7c5bd8-goog