On Wed, Aug 4, 2021 at 1:58 AM Oliver Upton <oupton@xxxxxxxxxx> 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 addresses a possible race in KVM_GET_CLOCK where
> use_master_clock is read outside of the pvclock_gtod_sync_lock.
>
> Patch 2 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 3-4 are some preparatory changes for exposing the TSC offset to
> userspace. Patch 5 provides a vCPU attribute to provide userspace access
> to the TSC offset.
>
> Patches 6-7 implement a test for the new additions to
> KVM_{GET,SET}_CLOCK.
>
> Patch 8 fixes some assertions in the kvm device attribute helpers.
>
> Patches 9-10 implement at test for the tsc offset attribute introduced in
> patch 5.
>
> Patches 11-12 lay the groundwork for patch 13, which exposes CNTVOFF_EL2
> through the ONE_REG interface.
>
> Patches 14-15 add test cases for userspace manipulation of the virtual
> counter-timer.
>
> Patches 16-17 add a vCPU attribute to adjust the host-guest offset of an
> ARM vCPU, but only implements support for ECV hosts. Patches 18-19 add
> support for non-ECV hosts by emulating physical counter offsetting.
>
> Patch 20 adds test cases for adjusting the host-guest offset, and
> finally patch 21 adds a test to measure the emulation overhead of
> CNTPCT_EL2.
>
> This series was tested on both an Ampere Mt. Jade and Haswell systems.
> Unfortunately, the ECV portions of this series are untested, as there is
> no ECV-capable hardware and the ARM fast models only partially implement
> ECV.
Small correction: I was only using the foundation model. Apparently
the AEM FVP provides full ECV support.
>
> Physical counter benchmark
> --------------------------
>
> The following data was collected by running 10000 iterations of the
> benchmark test from Patch 21 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 kvm/queue at the following commit:
>
> 6cd974485e25 ("KVM: selftests: Add a test of an unbacked nested PI descriptor")
>
> 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).
>
> v2 -> v3:
> - Added documentation for additions to KVM_{GET,SET}_CLOCK
> - Added documentation for all new vCPU attributes
> - Added documentation for suggested algorithm to migrate a guest's
> TSC(s)
> - Bug fixes throughout series
> - Rename KVM_CLOCK_REAL_TIME -> KVM_CLOCK_REALTIME
>
> v3 -> v4:
> - Added patch to address incorrect device helper assertions (Drew)
> - Carried Drew's r-b tags where appropriate
> - x86 selftest cleanup
> - Removed stale kvm_timer_init_vhe() function
> - Removed unnecessary GUEST_DONE() from selftests
>
> v4 -> v5:
> - Fix typo in TSC migration algorithm
> - Carry more of Drew's r-b tags
> - clean up run loop logic in counter emulation benchmark (missed from
> Drew's comments on v3)
>
> v5 -> v6:
> - Add fix for race in KVM_GET_CLOCK (Sean)
> - Fix 32-bit build issues in series + use of uninitialized host tsc
> value (Sean)
> - General style cleanups
> - Rework ARM virtual counter offsetting to match guest behavior. Use
> the ONE_REG interface instead of a VM attribute (Marc)
> - Maintain a single host-guest counter offset, which applies to both
> physical and virtual counters
> - Dropped some of Drew's r-b tags due to nontrivial patch changes
> (sorry for the churn!)
>
> v1: https://lore.kernel.org/kvm/20210608214742.1897483-1-oupton@xxxxxxxxxx/
> v2: https://lore.kernel.org/r/20210716212629.2232756-1-oupton@xxxxxxxxxx
> v3: https://lore.kernel.org/r/20210719184949.1385910-1-oupton@xxxxxxxxxx
> v4: https://lore.kernel.org/r/20210729001012.70394-1-oupton@xxxxxxxxxx
> v5: https://lore.kernel.org/r/20210729173300.181775-1-oupton@xxxxxxxxxx
>
> Oliver Upton (21):
> KVM: x86: Fix potential race in KVM_GET_CLOCK
> KVM: x86: Report host tsc and realtime values in KVM_GET_CLOCK
> KVM: x86: Take the pvclock sync lock behind the tsc_write_lock
> 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: Fix kvm device helper ioctl assertions
> selftests: KVM: Add helpers for vCPU device attributes
> selftests: KVM: Introduce system counter offset test
> KVM: arm64: Refactor update_vtimer_cntvoff()
> KVM: arm64: Separate guest/host counter offset values
> KVM: arm64: Allow userspace to configure a vCPU's virtual offset
> selftests: KVM: Add helper to check for register presence
> selftests: KVM: Add support for aarch64 to system_counter_offset_test
> arm64: cpufeature: Enumerate support for Enhanced Counter
> Virtualization
> KVM: arm64: Allow userspace to configure a guest's counter-timer
> offset
> KVM: arm64: Configure timer traps in vcpu_load() for VHE
> KVM: arm64: Emulate physical counter offsetting on non-ECV systems
> selftests: KVM: Test physical counter offsetting
> selftests: KVM: Add counter emulation benchmark
>
> Documentation/virt/kvm/api.rst | 52 ++-
> Documentation/virt/kvm/devices/vcpu.rst | 85 ++++
> Documentation/virt/kvm/locking.rst | 11 +
> arch/arm64/include/asm/kvm_asm.h | 2 +
> arch/arm64/include/asm/sysreg.h | 5 +
> arch/arm64/include/uapi/asm/kvm.h | 2 +
> arch/arm64/kernel/cpufeature.c | 10 +
> arch/arm64/kvm/arch_timer.c | 224 ++++++++++-
> arch/arm64/kvm/arm.c | 4 +-
> arch/arm64/kvm/guest.c | 6 +-
> arch/arm64/kvm/hyp/include/hyp/switch.h | 29 ++
> arch/arm64/kvm/hyp/nvhe/hyp-main.c | 6 +
> arch/arm64/kvm/hyp/nvhe/timer-sr.c | 16 +-
> arch/arm64/kvm/hyp/vhe/timer-sr.c | 5 +
> arch/arm64/tools/cpucaps | 1 +
> arch/x86/include/asm/kvm_host.h | 4 +
> arch/x86/include/uapi/asm/kvm.h | 4 +
> arch/x86/kvm/x86.c | 364 +++++++++++++-----
> include/clocksource/arm_arch_timer.h | 1 +
> include/kvm/arm_arch_timer.h | 6 +-
> 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 | 207 ++++++++++
> .../selftests/kvm/include/aarch64/processor.h | 24 ++
> .../testing/selftests/kvm/include/kvm_util.h | 13 +
> tools/testing/selftests/kvm/lib/kvm_util.c | 63 ++-
> .../kvm/system_counter_offset_test.c | 211 ++++++++++
> .../selftests/kvm/x86_64/kvm_clock_test.c | 204 ++++++++++
> 31 files changed, 1581 insertions(+), 143 deletions(-)
> 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
>
> --
> 2.32.0.605.g8dce9f2422-goog
>
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