Re: [PATCH v6 00/21] KVM: Add idempotent controls for migrating system counter state

[Oliver Upton <oupton@google.com>]

On Wed, Aug 4, 2021 at 4:05 AM Oliver Upton <oupton@google.com> wrote:
>
> On Wed, Aug 4, 2021 at 1:58 AM 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 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.

Ok. I've now tested this series on the FVP Base RevC fast model@v8.6 +
ECV=2. Passes on VHE, fails on nVHE.

I'll respin this series with the fix for nVHE+ECV soon.

--
Thanks,
Oliver

>
> >
> > 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@google.com/
> > v2: https://lore.kernel.org/r/20210716212629.2232756-1-oupton@google.com
> > v3: https://lore.kernel.org/r/20210719184949.1385910-1-oupton@google.com
> > v4: https://lore.kernel.org/r/20210729001012.70394-1-oupton@google.com
> > v5: https://lore.kernel.org/r/20210729173300.181775-1-oupton@google.com
> >
> > 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
> >