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.
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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