Re: [PATCH] KVM: optionally post write on ioeventfd write

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Mar 06, 2026 at 12:56:54PM +0000, Thanos Makatos wrote:
> Add a new flag, KVM_IOEVENTFD_FLAG_POST_WRITE, when assigning an
> ioeventfd that results in the value written by the guest to be copied
> to user-supplied memory instead of being discarded.
> 
> The goal of this new mechanism is to speed up doorbell writes on NVMe
> controllers emulated outside of the VMM. Currently, a doorbell write to
> an NVMe SQ tail doorbell requires returning from ioctl(KVM_RUN) and the
> VMM communicating the event, along with the doorbell value, to the NVMe
> controller emulation task.  With POST_WRITE, the NVMe emulation task is
> directly notified of the doorbell write and can find the doorbell value
> in a known location, without involving VMM.
> 
> Add tests for this new functionality.
> 
> LLM (claude-4.6-opus-high) was used mainly for the tests and to a
> lesser extent for pre-reviewing this patch.
> 
> Signed-off-by: Thanos Makatos <thanos.makatos@nutanix.com>
> ---
>  Documentation/virt/kvm/api.rst               |  13 +-
>  include/uapi/linux/kvm.h                     |   6 +-
>  tools/testing/selftests/kvm/Makefile.kvm     |   1 +
>  tools/testing/selftests/kvm/ioeventfd_test.c | 624 +++++++++++++++++++
>  virt/kvm/eventfd.c                           |  23 +
>  virt/kvm/kvm_main.c                          |   1 +
>  6 files changed, 666 insertions(+), 2 deletions(-)
>  create mode 100644 tools/testing/selftests/kvm/ioeventfd_test.c

Sean, Paolo: Ping

I would like to enable ioeventfd support in QEMU's NVMe device emulation
and this requires POST_WRITE for Windows guests (they don't support
NVMe's Doorbell Buffer Config feature so it's necessary to capture the
latest value written to the doorbell somehow).

POST_WRITE applies to any device that has a doorbell register where the
latest value needs to be captured. This looks like a reasonable
extension to ioeventfd.

I also looked into alternatives, like using userfaultfd, and didn't find
anything better.

Please consider merging this. Thanks!

Stefan

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