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

Marc Zyngier
2022-09-26T16:03:31Z

[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