[PATCH v6 00/21] KVM: Add idempotent controls for migrating system counter state
Oliver Upton
oupton at google.com
Mon Aug 9 17:04:38 PDT 2021
On Wed, Aug 4, 2021 at 3:03 PM Oliver Upton <oupton at google.com> wrote:
>
> On Wed, Aug 4, 2021 at 4:05 AM Oliver Upton <oupton at google.com> wrote:
> >
> > On Wed, Aug 4, 2021 at 1:58 AM Oliver Upton <oupton at 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.
Paolo,
Is there anything else you're waiting to see for the x86 portion of
this series after addressing Sean's comments? There's some work
remaining on the arm64 side, though I believe the two architectures
are now disjoint for this series.
--
Thanks,
Oliver
> > > 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 at 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
> > >
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