Re: [PATCH 04/10] KVM: selftests: Add KVM/PV clock selftest to prove timer correction

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On 18/04/2024 20:34, David Woodhouse wrote:
From: Jack Allister <jalliste@xxxxxxxxxx>

A VM's KVM/PV clock has an inherent relationship to its TSC (guest). When
either the host system live-updates or the VM is live-migrated this pairing
of the two clock sources should stay the same.

In reality this is not the case without some correction taking place. Two
perform a correction on the PVTI (PV time information) structure held by
KVM to effectively fixup the kvmclock_offset prior to the guest VM resuming
in either a live-update/migration scenario.

This test proves that without the necessary fixup there is a perceived
change in the guest TSC & KVM/PV clock relationship before and after a LU/
LM takes place.

The following steps are made to verify there is a delta in the relationship
and that it can be corrected:

1. PVTI is sampled by guest at boot (let's call this PVTI0).
2. Induce a change in PVTI data (KVM_REQ_MASTERCLOCK_UPDATE).
3. PVTI is sampled by guest after change (PVTI1).
4. Correction is requested by usermode to KVM using PVTI0.
5. PVTI is sampled by guest after correction (PVTI2).

The guest the records a singular TSC reference point in time and uses it to
calculate 3 KVM clock values utilizing the 3 recorded PVTI prior. Let's
call each clock value CLK[0-2].

In a perfect world CLK[0-2] should all be the same value if the KVM clock
& TSC relationship is preserved across the LU/LM (or faked in this test),
however it is not.

A delta can be observed between CLK0-CLK1 due to KVM recalculating the PVTI
(and the inaccuracies associated with that). A delta of ~3500ns can be
observed if guest TSC scaling to half host TSC frequency is also enabled,
where as without scaling this is observed at ~180ns.

With the correction it should be possible to achieve a delta of ±1ns.

An option to enable guest TSC scaling is available via invoking the tester
with -s/--scale-tsc.

Example of the test output below:
* selftests: kvm: pvclock_test
* scaling tsc from 2999999KHz to 1499999KHz
* before=5038374946 uncorrected=5038371437 corrected=5038374945
* delta_uncorrected=3509 delta_corrected=1

Signed-off-by: Jack Allister <jalliste@xxxxxxxxxx>
Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
CC: Paul Durrant <paul@xxxxxxx>
CC: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
  tools/testing/selftests/kvm/Makefile          |   1 +
  .../selftests/kvm/x86_64/pvclock_test.c       | 192 ++++++++++++++++++
  2 files changed, 193 insertions(+)
  create mode 100644 tools/testing/selftests/kvm/x86_64/pvclock_test.c

Reviewed-by: Paul Durrant <paul@xxxxxxx>

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