Re: [PATCH 2/2] KVM: selftests: Add KVM/PV clock selftest to prove timer drift correction

[Chen, Zide]

On 4/8/2024 3:07 PM, Jack Allister wrote:
> This test proves that there is an inherent KVM/PV clock drift away from the
> guest TSC when KVM decides to update the PV time information structure due
> to a KVM_REQ_MASTERCLOCK_UPDATE. This drift is exascerbated when a guest is
> using TSC scaling and running at a different frequency to the host TSC [1].
> It also proves that KVM_[GS]ET_CLOCK_GUEST API is working to mitigate the
> drift from TSC to within ±1ns.
> 
> The test simply records the PVTI (PV time information) at time of guest
> creation, after KVM has updated it's mapped PVTI structure and once the
> correction has taken place.
> 
> A singular point in time is then recorded via the guest TSC and is used to
> calculate the a PV clock value using each of the 3 PVTI structures.
> 
> As seen below a drift of ~3500ns is observed if no correction has taken
> place after KVM has updated the PVTI via master clock update. However,
> after the correction a delta of at most 1ns can be seen.
> 
> * selftests: kvm: pvclock_test
> * scaling tsc from 2999999KHz to 1499999KHz
> * before=5038374946 uncorrected=5038371437 corrected=5038374945
> * delta_uncorrected=3509 delta_corrected=1
> 
> Clocksource check code has been borrowed from [2].
> 
> [1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=451a707813ae
> [2]: https://lore.kernel.org/kvm/20240106083346.29180-1-dongli.zhang@xxxxxxxxxx/
> 
> Signed-off-by: Jack Allister <jalliste@xxxxxxxxxx>
> CC: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
> CC: Paul Durrant <paul@xxxxxxx>
> ---
>  tools/testing/selftests/kvm/Makefile          |   1 +
>  .../selftests/kvm/x86_64/pvclock_test.c       | 223 ++++++++++++++++++
>  2 files changed, 224 insertions(+)
>  create mode 100644 tools/testing/selftests/kvm/x86_64/pvclock_test.c
> 
> diff --git a/tools/testing/selftests/kvm/Makefile b/tools/testing/selftests/kvm/Makefile
> index 741c7dc16afc..02ee1205bbed 100644
> --- a/tools/testing/selftests/kvm/Makefile
> +++ b/tools/testing/selftests/kvm/Makefile
> @@ -87,6 +87,7 @@ TEST_GEN_PROGS_x86_64 += x86_64/pmu_counters_test
>  TEST_GEN_PROGS_x86_64 += x86_64/pmu_event_filter_test
>  TEST_GEN_PROGS_x86_64 += x86_64/private_mem_conversions_test
>  TEST_GEN_PROGS_x86_64 += x86_64/private_mem_kvm_exits_test
> +TEST_GEN_PROGS_x86_64 += x86_64/pvclock_test
>  TEST_GEN_PROGS_x86_64 += x86_64/set_boot_cpu_id
>  TEST_GEN_PROGS_x86_64 += x86_64/set_sregs_test
>  TEST_GEN_PROGS_x86_64 += x86_64/smaller_maxphyaddr_emulation_test
> diff --git a/tools/testing/selftests/kvm/x86_64/pvclock_test.c b/tools/testing/selftests/kvm/x86_64/pvclock_test.c
> new file mode 100644
> index 000000000000..172ef4d19c60
> --- /dev/null
> +++ b/tools/testing/selftests/kvm/x86_64/pvclock_test.c
> @@ -0,0 +1,223 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Copyright © 2024, Amazon.com, Inc. or its affiliates.
> + *
> + * Tests for pvclock API
> + * KVM_SET_CLOCK_GUEST/KVM_GET_CLOCK_GUEST
> + */
> +#include <asm/pvclock.h>
> +#include <asm/pvclock-abi.h>
> +#include <sys/stat.h>
> +#include <stdint.h>
> +#include <stdio.h>
> +
> +#include "test_util.h"
> +#include "kvm_util.h"
> +#include "processor.h"
> +
> +enum {
> +	STAGE_FIRST_BOOT,
> +	STAGE_UNCORRECTED,
> +	STAGE_CORRECTED,
> +	NUM_STAGES
> +};
> +
> +#define KVMCLOCK_GPA 0xc0000000ull
> +#define KVMCLOCK_SIZE sizeof(struct pvclock_vcpu_time_info)
> +
> +static void trigger_pvti_update(vm_paddr_t pvti_pa)
> +{
> +	/*
> +	 * We need a way to trigger KVM to update the fields
> +	 * in the PV time info. The easiest way to do this is
> +	 * to temporarily switch to the old KVM system time
> +	 * method and then switch back to the new one.
> +	 */
> +	wrmsr(MSR_KVM_SYSTEM_TIME, pvti_pa | KVM_MSR_ENABLED);
> +	wrmsr(MSR_KVM_SYSTEM_TIME_NEW, pvti_pa | KVM_MSR_ENABLED);
> +}
> +
> +static void guest_code(vm_paddr_t pvti_pa)
> +{
> +	struct pvclock_vcpu_time_info *pvti_va =
> +		(struct pvclock_vcpu_time_info *)pvti_pa;
> +
> +	struct pvclock_vcpu_time_info pvti_boot;
> +	struct pvclock_vcpu_time_info pvti_uncorrected;
> +	struct pvclock_vcpu_time_info pvti_corrected;
> +	uint64_t cycles_boot;
> +	uint64_t cycles_uncorrected;
> +	uint64_t cycles_corrected;
> +	uint64_t tsc_guest;
> +
> +	/*
> +	 * Setup the KVMCLOCK in the guest & store the original
> +	 * PV time structure that is used.
> +	 */
> +	wrmsr(MSR_KVM_SYSTEM_TIME_NEW, pvti_pa | KVM_MSR_ENABLED);
> +	pvti_boot = *pvti_va;
> +	GUEST_SYNC(STAGE_FIRST_BOOT);
> +
> +	/*
> +	 * Trigger an update of the PVTI, if we calculate
> +	 * the KVM clock using this structure we'll see
> +	 * a drift from the TSC.
> +	 */
> +	trigger_pvti_update(pvti_pa);
> +	pvti_uncorrected = *pvti_va;
> +	GUEST_SYNC(STAGE_UNCORRECTED);
> +
> +	/*
> +	 * The test should have triggered the correction by this
> +	 * point in time. We have a copy of each of the PVTI structs
> +	 * at each stage now.
> +	 *
> +	 * Let's sample the timestamp at a SINGLE point in time and
> +	 * then calculate what the KVM clock would be using the PVTI
> +	 * from each stage.
> +	 *
> +	 * Then return each of these values to the tester.
> +	 */
> +	pvti_corrected = *pvti_va;
> +	tsc_guest = rdtsc();
> +
> +	cycles_boot = __pvclock_read_cycles(&pvti_boot, tsc_guest);
> +	cycles_uncorrected = __pvclock_read_cycles(&pvti_uncorrected, tsc_guest);
> +	cycles_corrected = __pvclock_read_cycles(&pvti_corrected, tsc_guest);
> +
> +	GUEST_SYNC_ARGS(STAGE_CORRECTED, cycles_boot, cycles_uncorrected,
> +			cycles_corrected, 0);
> +}
> +
> +static void run_test(struct kvm_vm *vm, struct kvm_vcpu *vcpu)
> +{
> +	struct ucall uc;
> +	uint64_t ucall_reason;
> +	struct pvclock_vcpu_time_info pvti_before;
> +	uint64_t before, uncorrected, corrected;
> +	int64_t delta_uncorrected, delta_corrected;
> +
> +	/* Loop through each stage of the test. */
> +	while (true) {
> +
> +		/* Start/restart the running vCPU code. */
> +		vcpu_run(vcpu);
> +		TEST_ASSERT_KVM_EXIT_REASON(vcpu, KVM_EXIT_IO);
> +
> +		/* Retrieve and verify our stage. */
> +		ucall_reason = get_ucall(vcpu, &uc);
> +		TEST_ASSERT(ucall_reason == UCALL_SYNC,
> +			    "Unhandled ucall reason=%lu",
> +			    ucall_reason);
> +
> +		/* Run host specific code relating to stage. */
> +		switch (uc.args[1]) {
> +		case STAGE_FIRST_BOOT:
> +			/* Store the KVM clock values before an update. */
> +			vm_ioctl(vm, KVM_GET_CLOCK_GUEST, &pvti_before);
> +
> +			/* Sleep for a set amount of time to induce drift. */
> +			sleep(5);
> +			break;
> +
> +		case STAGE_UNCORRECTED:
> +			/* Restore the KVM clock values. */
> +			vm_ioctl(vm, KVM_SET_CLOCK_GUEST, &pvti_before);
> +			break;
> +
> +		case STAGE_CORRECTED:
> +			/* Query the clock information and verify delta. */
> +			before = uc.args[2];
> +			uncorrected = uc.args[3];
> +			corrected = uc.args[4];
> +
> +			delta_uncorrected = before - uncorrected;
> +			delta_corrected = before - corrected;
> +
> +			pr_info("before=%lu uncorrected=%lu corrected=%lu\n",
> +				before, uncorrected, corrected);
> +
> +			pr_info("delta_uncorrected=%ld delta_corrected=%ld\n",
> +				delta_uncorrected, delta_corrected);
> +
> +			TEST_ASSERT((delta_corrected <= 1) && (delta_corrected >= -1),
> +				    "larger than expected delta detected = %ld", delta_corrected);

I'm wondering what's the underling theory that we definitely can achieve
±1ns accuracy? I tested it on a Sapphire Rapids @2100MHz TSC frequency,
and I can see delta_corrected=2 in ~2% cases.