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

On Wed, Jul 21, 2021 at 8:28 AM Andrew Jones [off-list ref] wrote:

On Fri, Jul 16, 2021 at 09:26:17PM +0000, Oliver Upton wrote:

quoted

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 -> 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).

Oops, I see there's a v3 of this series. I'll switch to reviewing that. I
think my comments / r-b's apply to that version as well though.

Hey Drew,

Thanks for the review. I'll address your comments from both v2 and v3
in the next series.

--
Thanks,
Oliver

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