[PATCH 00/10] KVM: Add idempotent controls for migrating system counter state

[Oliver Upton <oupton@google.com>]

KVM's current means of saving/restoring system counters is plagued with
temporal issues. At least on ARM64 and x86, we migrate the guest's
system counter by-value through the respective guest system register
values (cntvct_el0, ia32_tsc). Restoring system counters by-value is
brittle as the state is not idempotent: the host system counter is still
oscillating between the attempted save and restore. Furthermore, VMMs
may wish to transparently live migrate guest VMs, meaning that they
include the elapsed time due to live migration blackout in the guest
system counter view. The VMM thread could be preempted for any number of
reasons (scheduler, L0 hypervisor under nested) between the time that
it calculates the desired guest counter value and when KVM actually sets
this counter state.

Despite the value-based interface that we present to userspace, KVM
actually has idempotent guest controls by way of system counter offsets.
We can avoid all of the issues associated with a value-based interface
by abstracting these offset controls in new ioctls. This series
introduces KVM_{GET,SET}_SYSTEM_COUNTER_STATE ioctls, meant to provide
userspace with idempotent controls of the guest system counter.

Patch 1 defines the ioctls, and was separated from the two provided
implementations for the sake of review. If it is more intuitive, this
patch can be squashed into the implementation commit.

Patch 2 realizes initial support for ARM64, migrating only the state
associated with the guest's virtual counter-timer. Patch 3 introduces a
KVM selftest to assert that userspace manipulation via the
aforementioned ioctls produces the expected system counter values within
the guest.

Patch 4 extends upon the ARM64 implementation by adding support for
physical counter-timer offsetting. This is currently backed by a
trap-and-emulate implementation, but can also be virtualized in hardware
that fully implements ARMv8.6-ECV. ECV support has been elided from this
series out of convenience for the author :) Patch 5 adds some test cases
to the newly-minted kvm selftest to validate expectations of physical
counter-timer emulation.

Patch 6 introduces yet another KVM selftest for aarch64, intended to
measure the effects of physical counter-timer emulation. Data for this
test can be found below, but basically there is some tradeoff of
overhead for the sake of correctness, but it isn't too bad.

Patches 7-8 add support for the ioctls to x86 by shoehorning the
controls into the pre-existing synchronization heuristics. Patch 7
provides necessary helper methods for the implementation to play nice
with those heuristics, and patch 8 actually implements the ioctls.

Patch 9 adds x86 test cases to the system counter KVM selftest. Lastly,
patch 10 documents the ioctls for both x86 and arm64.

All patches apply cleanly to kvm/next at the following commit:

a4345a7cecfb ("Merge tag 'kvmarm-fixes-5.13-1' of git://git.kernel.org/pub/scm/linux/kernel/git/kvmarm/kvmarm into HEAD")

Physical counter benchmark
--------------------------

The following data was collected by running 10000 iterations of the
benchmark test from Patch 6 on an Ampere Mt. Jade reference server, A 2S
machine with 2 80-core Ampere Altra SoCs. Measurements were collected
for both VHE and nVHE operation using the `kvm-arm.mode=` command-line
parameter.

nVHE
----

+--------------------+--------+---------+
|       Metric       | Native | Trapped |
+--------------------+--------+---------+
| Average            | 54ns   | 148ns   |
| Standard Deviation | 124ns  | 122ns   |
| 95th Percentile    | 258ns  | 348ns   |
+--------------------+--------+---------+

VHE
---

+--------------------+--------+---------+
|       Metric       | Native | Trapped |
+--------------------+--------+---------+
| Average            | 53ns   | 152ns   |
| Standard Deviation | 92ns   | 94ns    |
| 95th Percentile    | 204ns  | 307ns   |
+--------------------+--------+---------+

Oliver Upton (10):
  KVM: Introduce KVM_{GET,SET}_SYSTEM_COUNTER_STATE ioctls
  KVM: arm64: Implement initial support for KVM_CAP_SYSTEM_COUNTER_STATE
  selftests: KVM: Introduce system_counter_state_test
  KVM: arm64: Add userspace control of the guest's physical counter
  selftests: KVM: Add test cases for physical counter offsetting
  selftests: KVM: Add counter emulation benchmark
  KVM: x86: Refactor tsc synchronization code
  KVM: x86: Implement KVM_CAP_SYSTEM_COUNTER_STATE
  selftests: KVM: Add support for x86 to system_counter_state_test
  Documentation: KVM: Document KVM_{GET,SET}_SYSTEM_COUNTER_STATE ioctls

 Documentation/virt/kvm/api.rst                |  98 +++++++
 Documentation/virt/kvm/locking.rst            |  11 +
 arch/arm64/include/asm/kvm_host.h             |   6 +
 arch/arm64/include/asm/sysreg.h               |   1 +
 arch/arm64/include/uapi/asm/kvm.h             |  17 ++
 arch/arm64/kvm/arch_timer.c                   |  84 +++++-
 arch/arm64/kvm/arm.c                          |  25 ++
 arch/arm64/kvm/hyp/include/hyp/switch.h       |  31 +++
 arch/arm64/kvm/hyp/nvhe/timer-sr.c            |  16 +-
 arch/x86/include/asm/kvm_host.h               |   1 +
 arch/x86/include/uapi/asm/kvm.h               |   8 +
 arch/x86/kvm/x86.c                            | 176 +++++++++---
 include/uapi/linux/kvm.h                      |   5 +
 tools/testing/selftests/kvm/.gitignore        |   2 +
 tools/testing/selftests/kvm/Makefile          |   3 +
 .../kvm/aarch64/counter_emulation_benchmark.c | 209 ++++++++++++++
 .../selftests/kvm/include/aarch64/processor.h |  24 ++
 .../selftests/kvm/system_counter_state_test.c | 256 ++++++++++++++++++
 18 files changed, 926 insertions(+), 47 deletions(-)
 create mode 100644 tools/testing/selftests/kvm/aarch64/counter_emulation_benchmark.c
 create mode 100644 tools/testing/selftests/kvm/system_counter_state_test.c

-- 
2.32.0.rc1.229.g3e70b5a671-goog