Re: [PATCH v2 00/12] KVM: Add idempotent controls for migrating system counter state

[Oliver Upton <oupton@google.com>]

On Fri, Jul 16, 2021 at 2:26 PM Oliver Upton <oupton@google.com> wrote:
>
> 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 new vCPU device attributes to provide userspace access to the
> vCPU's system counter offset.
>
> Patch 1 adopts Paolo's suggestion, augmenting the KVM_{GET,SET}_CLOCK
> ioctls to provide userspace with a (host_tsc, realtime) instant. This is
> essential for a VMM to perform precise migration of the guest's system
> counters.
>
> Patches 2-3 add support for x86 by shoehorning the new controls into the
> pre-existing synchronization heuristics.
>
> Patches 4-5 implement a test for the new additions to
> KVM_{GET,SET}_CLOCK.
>
> Patches 6-7 implement at test for the tsc offset attribute introduced in
> patch 3.
>
> Patch 8 adds a device attribute for the arm64 virtual counter-timer
> offset.
>
> Patch 9 extends the test from patch 7 to cover the arm64 virtual
> counter-timer offset.
>
> Patch 10 adds a device attribute for the arm64 physical counter-timer
> offset. Currently, this is implemented as a synthetic register, forcing
> the guest to trap to the host and emulating the offset in the fast exit
> path. Later down the line we will have hardware with FEAT_ECV, which
> allows the hypervisor to perform physical counter-timer offsetting in
> hardware (CNTPOFF_EL2).
>
> Patch 11 extends the test from patch 7 to cover the arm64 physical
> counter-timer offset.
>
> Patch 12 introduces a benchmark to measure the overhead of emulation in
> patch 10.
>
> 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   |
> +--------------------+--------+---------+
>
> This series applies cleanly to the following commit:
>
> 1889228d80fe ("KVM: selftests: smm_test: Test SMM enter from L2")

v1: https://lore.kernel.org/kvm/20210608214742.1897483-1-oupton@google.com/

> v1 -> v2:
>   - Reimplemented as vCPU device attributes instead of a distinct ioctl.
>   - Added the (realtime, host_tsc) instant support to
>     KVM_{GET,SET}_CLOCK
>   - Changed the arm64 implementation to broadcast counter offset values
>     to all vCPUs in a guest. This upholds the architectural expectations
>     of a consistent counter-timer across CPUs.
>   - Fixed a bug with traps in VHE mode. We now configure traps on every
>     transition into a guest to handle differing VMs (trapped, emulated).
>
> Oliver Upton (12):
>   KVM: x86: Report host tsc and realtime values in KVM_GET_CLOCK
>   KVM: x86: Refactor tsc synchronization code
>   KVM: x86: Expose TSC offset controls to userspace
>   tools: arch: x86: pull in pvclock headers
>   selftests: KVM: Add test for KVM_{GET,SET}_CLOCK
>   selftests: KVM: Add helpers for vCPU device attributes
>   selftests: KVM: Introduce system counter offset test
>   KVM: arm64: Allow userspace to configure a vCPU's virtual offset
>   selftests: KVM: Add support for aarch64 to system_counter_offset_test
>   KVM: arm64: Provide userspace access to the physical counter offset
>   selftests: KVM: Test physical counter offsetting
>   selftests: KVM: Add counter emulation benchmark
>
>  Documentation/virt/kvm/api.rst                |  42 +-
>  Documentation/virt/kvm/locking.rst            |  11 +
>  arch/arm64/include/asm/kvm_host.h             |   1 +
>  arch/arm64/include/asm/kvm_hyp.h              |   2 -
>  arch/arm64/include/asm/sysreg.h               |   1 +
>  arch/arm64/include/uapi/asm/kvm.h             |   2 +
>  arch/arm64/kvm/arch_timer.c                   | 118 ++++-
>  arch/arm64/kvm/arm.c                          |   4 +-
>  arch/arm64/kvm/hyp/include/hyp/switch.h       |  23 +
>  arch/arm64/kvm/hyp/include/hyp/timer-sr.h     |  26 ++
>  arch/arm64/kvm/hyp/nvhe/switch.c              |   2 -
>  arch/arm64/kvm/hyp/nvhe/timer-sr.c            |  21 +-
>  arch/arm64/kvm/hyp/vhe/timer-sr.c             |  27 ++
>  arch/x86/include/asm/kvm_host.h               |   4 +
>  arch/x86/include/uapi/asm/kvm.h               |   4 +
>  arch/x86/kvm/x86.c                            | 421 ++++++++++++++----
>  include/kvm/arm_arch_timer.h                  |   2 -
>  include/uapi/linux/kvm.h                      |   7 +-
>  tools/arch/x86/include/asm/pvclock-abi.h      |  48 ++
>  tools/arch/x86/include/asm/pvclock.h          | 103 +++++
>  tools/testing/selftests/kvm/.gitignore        |   3 +
>  tools/testing/selftests/kvm/Makefile          |   4 +
>  .../kvm/aarch64/counter_emulation_benchmark.c | 215 +++++++++
>  .../selftests/kvm/include/aarch64/processor.h |  24 +
>  .../testing/selftests/kvm/include/kvm_util.h  |  11 +
>  tools/testing/selftests/kvm/lib/kvm_util.c    |  38 ++
>  .../kvm/system_counter_offset_test.c          | 206 +++++++++
>  .../selftests/kvm/x86_64/kvm_clock_test.c     | 210 +++++++++
>  28 files changed, 1447 insertions(+), 133 deletions(-)
>  create mode 100644 arch/arm64/kvm/hyp/include/hyp/timer-sr.h
>  create mode 100644 tools/arch/x86/include/asm/pvclock-abi.h
>  create mode 100644 tools/arch/x86/include/asm/pvclock.h
>  create mode 100644 tools/testing/selftests/kvm/aarch64/counter_emulation_benchmark.c
>  create mode 100644 tools/testing/selftests/kvm/system_counter_offset_test.c
>  create mode 100644 tools/testing/selftests/kvm/x86_64/kvm_clock_test.c
>
> --
> 2.32.0.402.g57bb445576-goog
>