[PATCH v2 0/6] KVM: Fix dirty-ring ordering on weakly ordered architectures

[Marc Zyngier <maz@kernel.org>]

[Same distribution list as Gavin's dirty-ring on arm64 series]

This is an update on the initial series posted as [0].

As Gavin started posting patches enabling the dirty-ring infrastructure
on arm64 [1], it quickly became apparent that the API was never intended
to work on relaxed memory ordering architectures (owing to its x86
origins).

This series tries to retrofit some ordering into the existing API by:

- relying on acquire/release semantics which are the default on x86,
  but need to be explicit on arm64

- adding a new capability that indicate which flavor is supported, either
  with explicit ordering (arm64) or both implicit and explicit (x86),
  as suggested by Paolo at KVM Forum

- documenting the requirements for this new capability on weakly ordered
  architectures

- updating the selftests to do the right thing

Ideally, this series should be a prefix of Gavin's, plus a small change
to his series:

diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig
index 0309b2d0f2da..7785379c5048 100644
--- a/arch/arm64/kvm/Kconfig
+++ b/arch/arm64/kvm/Kconfig
@@ -32,7 +32,7 @@ menuconfig KVM
 	select KVM_VFIO
 	select HAVE_KVM_EVENTFD
 	select HAVE_KVM_IRQFD
-	select HAVE_KVM_DIRTY_RING
+	select HAVE_KVM_DIRTY_RING_ACQ_REL
 	select HAVE_KVM_MSI
 	select HAVE_KVM_IRQCHIP
 	select HAVE_KVM_IRQ_ROUTING

This has been very lightly tested on an arm64 box with Gavin's v3 [2] series.

* From v1:
  - Repainted the config symbols and new capability so that their
    naming is more acceptable and causes less churn
  - Fixed a couple of blunders as pointed out by Peter and Paolo
  - Updated the documentation

[0] https://lore.kernel.org/r/20220922170133.2617189-1-maz@kernel.org
[1] https://lore.kernel.org/lkml/YyiV%2Fl7O23aw5aaO@xz-m1.local/T/
[2] https://lore.kernel.org/r/20220922003214.276736-1-gshan@redhat.com

Marc Zyngier (6):
  KVM: Use acquire/release semantics when accessing dirty ring GFN state
  KVM: Add KVM_CAP_DIRTY_LOG_RING_ACQ_REL capability and config option
  KVM: x86: Select CONFIG_HAVE_KVM_DIRTY_RING_ACQ_REL
  KVM: Document weakly ordered architecture requirements for dirty ring
  KVM: selftests: dirty-log: Upgrade flag accesses to acquire/release
    semantics
  KVM: selftests: dirty-log: Use KVM_CAP_DIRTY_LOG_RING_ACQ_REL if
    available

 Documentation/virt/kvm/api.rst               | 17 +++++++++++++++--
 arch/x86/kvm/Kconfig                         |  3 ++-
 include/uapi/linux/kvm.h                     |  1 +
 tools/testing/selftests/kvm/dirty_log_test.c |  8 +++++---
 tools/testing/selftests/kvm/lib/kvm_util.c   |  5 ++++-
 virt/kvm/Kconfig                             | 14 ++++++++++++++
 virt/kvm/dirty_ring.c                        |  4 ++--
 virt/kvm/kvm_main.c                          |  9 ++++++++-
 8 files changed, 51 insertions(+), 10 deletions(-)

-- 
2.34.1