[PATCH v3 0/6] KVM: Improve MMIO Coalescing API

[Ilias Stamatis <ilstam@amazon.com>]

The current MMIO coalescing design has a few drawbacks which limit its
usefulness. Currently all coalesced MMIO zones use the same ring buffer.
That means that upon a userspace exit we have to handle potentially
unrelated MMIO writes synchronously. And a VM-wide lock needs to be
taken in the kernel when an MMIO exit occurs.

Additionally, there is no direct way for userspace to be notified about
coalesced MMIO writes. If the next MMIO exit to userspace is when the
ring buffer has filled then a substantial (and unbounded) amount of time
may have passed since the first coalesced MMIO.

This series adds new ioctls to KVM that allow for greater control by
making it possible to associate different MMIO zones with different ring
buffers. It also allows userspace to use poll() to check for coalesced
writes in order to avoid userspace exits in vCPU threads (see patch 3
for why this can be useful).

The idea of improving the API in this way originally came from Paul
Durrant (pdurrant@amazon.co.uk) but the implementation is mine.

The first patch in the series is a bug in the existing code that I
discovered while writing a selftest and can be merged independently.

Changelog:

v3:
  - Fixed test robot build warning and made small changes suggested by
    Sean Christopherson

v2: https://lore.kernel.org/kvm/20240718193543.624039-1-ilstam@amazon.com/T/

v1: https://lore.kernel.org/kvm/20240710085259.2125131-1-ilstam@amazon.com/T/

Ilias Stamatis (6):
  KVM: Fix coalesced_mmio_has_room()
  KVM: Add KVM_CREATE_COALESCED_MMIO_BUFFER ioctl
  KVM: Support poll() on coalesced mmio buffer fds
  KVM: Add KVM_(UN)REGISTER_COALESCED_MMIO2 ioctls
  KVM: Documentation: Document v2 of coalesced MMIO API
  KVM: selftests: Add coalesced_mmio_test

 Documentation/virt/kvm/api.rst                |  91 +++++
 include/linux/kvm_host.h                      |   1 +
 include/uapi/linux/kvm.h                      |  18 +
 tools/testing/selftests/kvm/Makefile          |   1 +
 .../selftests/kvm/coalesced_mmio_test.c       | 313 ++++++++++++++++++
 virt/kvm/coalesced_mmio.c                     | 203 +++++++++++-
 virt/kvm/coalesced_mmio.h                     |  17 +-
 virt/kvm/kvm_main.c                           |  37 ++-
 8 files changed, 661 insertions(+), 20 deletions(-)
 create mode 100644 tools/testing/selftests/kvm/coalesced_mmio_test.c

-- 
2.34.1