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

[Paul Durrant]

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
> new IOCTLs (KVM_GET_CLOCK_GUEST/KVM_SET_CLOCK_GUEST) can be utilized to
> 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>