Re: [PATCH v7 34/36] KVM: selftests: Add master clock offset test

On Tue, Jul 28, 2026, David Woodhouse wrote:
> From: David Woodhouse <[email protected]>
>=20
> Verify that KVM master clock mode remains active when vCPUs have
> different TSC offsets but the same frequency. Creates three vCPUs,
> sets one to a different TSC value, and confirms:
>=20
>  - KVM_CLOCK_HOST_TSC is set (master clock active)
>  - KVM_CLOCK_TSC_STABLE IS set (master clock =E2=87=92 stable flag)
>  - PVCLOCK_TSC_STABLE_BIT is NOT set (offsets differ)
>=20
> Signed-off-by: David Woodhouse <[email protected]>
> Assisted-by: Kiro (claude-opus-4.6-1m)
> ---
>  tools/testing/selftests/kvm/Makefile.kvm      |   1 +
>  .../kvm/x86/masterclock_offset_test.c         | 180 ++++++++++++++++++
>  2 files changed, 181 insertions(+)
>  create mode 100644 tools/testing/selftests/kvm/x86/masterclock_offset_te=
st.c
>=20
> diff --git a/tools/testing/selftests/kvm/Makefile.kvm b/tools/testing/sel=
ftests/kvm/Makefile.kvm
> index 95539deea266..f6027a35f790 100644
> --- a/tools/testing/selftests/kvm/Makefile.kvm
> +++ b/tools/testing/selftests/kvm/Makefile.kvm
> @@ -108,6 +108,7 @@ TEST_GEN_PROGS_x86 +=3D x86/pmu_event_filter_test
>  TEST_GEN_PROGS_x86 +=3D x86/private_mem_conversions_test
>  TEST_GEN_PROGS_x86 +=3D x86/private_mem_kvm_exits_test
>  TEST_GEN_PROGS_x86 +=3D x86/pvclock_test
> +TEST_GEN_PROGS_x86 +=3D x86/masterclock_offset_test

Keep these sorted alphabetically.

>  TEST_GEN_PROGS_x86 +=3D x86/pvclock_migration_test
>  TEST_GEN_PROGS_x86 +=3D x86/set_boot_cpu_id
>  TEST_GEN_PROGS_x86 +=3D x86/set_sregs_test
> diff --git a/tools/testing/selftests/kvm/x86/masterclock_offset_test.c b/=
tools/testing/selftests/kvm/x86/masterclock_offset_test.c
> new file mode 100644
> index 000000000000..88e2bd2edab5
> --- /dev/null
> +++ b/tools/testing/selftests/kvm/x86/masterclock_offset_test.c
> @@ -0,0 +1,180 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Test that KVM master clock mode works with different TSC offsets
> + * as long as all vCPUs have the same TSC frequency.
> + */
> +#include <stdint.h>
> +#include <string.h>
> +
> +#include "test_util.h"
> +#include "kvm_util.h"
> +#include "processor.h"
> +
> +#include <asm/pvclock-abi.h>
> +
> +#define KVMCLOCK_GPA	0xc0000000ull
> +#define TSC_OFFSET	(1000000000ULL)
> +
> +static uint64_t pvclock_calc(struct pvclock_vcpu_time_info *pvti, uint64=
_t guest_tsc)
> +{
> +	uint64_t delta =3D guest_tsc - pvti->tsc_timestamp;
> +
> +	if (pvti->tsc_shift >=3D 0)
> +		delta <<=3D pvti->tsc_shift;
> +	else
> +		delta >>=3D -(int)pvti->tsc_shift;
> +
> +	return pvti->system_time + ((__uint128_t)delta * pvti->tsc_to_system_mu=
l >> 32);
> +}
> +
> +static void guest_code(void)
> +{
> +	wrmsr(MSR_KVM_SYSTEM_TIME_NEW, KVMCLOCK_GPA | KVM_MSR_ENABLED);
> +	for (;;)
> +		GUEST_SYNC(0);
> +}
> +
> +int main(void)
> +{
> +	struct kvm_vcpu *vcpus[3];
> +	struct kvm_clock_data clock;
> +	struct pvclock_vcpu_time_info pvti[3];
> +	struct kvm_vm *vm;
> +	uint64_t offset0, host_tsc, clk0, clk2;
> +	int i;
> +
> +	TEST_REQUIRE(sys_clocksource_is_based_on_tsc());
> +
> +	vm =3D vm_create_with_vcpus(3, guest_code, vcpus);
> +
> +	TEST_REQUIRE(!__vcpu_has_device_attr(vcpus[0], KVM_VCPU_TSC_CTRL,
> +					     KVM_VCPU_TSC_OFFSET));
> +
> +	vm_userspace_mem_region_add(vm, VM_MEM_SRC_ANONYMOUS,
> +				    KVMCLOCK_GPA, 1,
> +				    vm_calc_num_guest_pages(VM_MODE_DEFAULT,
> +							   getpagesize()), 0);
> +	virt_map(vm, KVMCLOCK_GPA, KVMCLOCK_GPA,
> +		 vm_calc_num_guest_pages(VM_MODE_DEFAULT, getpagesize()));
> +
> +	/* Get vCPU 0's default offset and set vCPU 2's offset higher */
> +	vcpu_device_attr_get(vcpus[0], KVM_VCPU_TSC_CTRL,
> +			     KVM_VCPU_TSC_OFFSET, &offset0);
> +	uint64_t offset2 =3D offset0 + TSC_OFFSET;
> +	vcpu_device_attr_set(vcpus[2], KVM_VCPU_TSC_CTRL,
> +			     KVM_VCPU_TSC_OFFSET, &offset2);
> +
> +	/* Run each vCPU to enable kvmclock (with offset already set) */
> +	for (i =3D 0; i < 3; i++) {

ARRAY_SIZE(vcpus)

> +		vcpu_run(vcpus[i]);
> +		TEST_ASSERT_KVM_EXIT_REASON(vcpus[i], KVM_EXIT_IO);
> +	}
> +
> +	/* Check master clock is active */
> +	memset(&clock, 0, sizeof(clock));
> +	vm_ioctl(vm, KVM_GET_CLOCK, &clock);

Provide an API.

> +	pr_info("KVM_GET_CLOCK flags: 0x%x\n", clock.flags);
> +	TEST_ASSERT(clock.flags & KVM_CLOCK_HOST_TSC,
> +		    "Master clock should be active, flags=3D0x%x", clock.flags);
> +	TEST_ASSERT(clock.flags & KVM_CLOCK_TSC_STABLE,
> +		    "KVM_CLOCK_TSC_STABLE should be set, flags=3D0x%x", clock.flags);
> +
> +	/* Get per-vCPU pvclock in order 0, 2, 1 */
> +	int order[] =3D {0, 2, 1};

Please don't declare variables in the middle code.  Yes, it's now technical=
ly
possible thanks to all the guard() magic, but it's still against preferred =
kernel
style.

Though this itself is probably a really, really good indication that this n=
eeds
to be split into multiple testcases.  Maybe using TEST_F() and a custom fix=
ture?