guest_tsc = host_tsc + KVM_VCPU_TSC_OFFSET
-This attribute is useful for the precise migration of a guest's TSC. The
-following describes a possible algorithm to use for the migration of a
-guest's TSC:
+This attribute is useful to adjust the guest's TSC on live migration,
+so that the TSC counts the time during which the VM was paused. The
+following describes a possible algorithm to use for this purpose.
From the source VMM process:
-1. Invoke the KVM_GET_CLOCK ioctl to record the host TSC (t_0),
- kvmclock nanoseconds (k_0), and realtime nanoseconds (r_0).
+1. Invoke the KVM_GET_CLOCK ioctl to record the host TSC (tsc_src),
+ kvmclock nanoseconds (guest_src), and host CLOCK_REALTIME nanoseconds
+ (host_src).
2. Read the KVM_VCPU_TSC_OFFSET attribute for every vCPU to record the
- guest TSC offset (off_n).
+ guest TSC offset (ofs_src[i]).
3. Invoke the KVM_GET_TSC_KHZ ioctl to record the frequency of the
guest's TSC (freq).
From the destination VMM process:
-4. Invoke the KVM_SET_CLOCK ioctl, providing the kvmclock nanoseconds
- (k_0) and realtime nanoseconds (r_0) in their respective fields.
- Ensure that the KVM_CLOCK_REALTIME flag is set in the provided
- structure. KVM will advance the VM's kvmclock to account for elapsed
- time since recording the clock values.
+4. Invoke the KVM_SET_CLOCK ioctl, providing the source nanoseconds from
+ kvmclock (guest_src) and CLOCK_REALTIME (host_src) in their respective
+ fields. Ensure that the KVM_CLOCK_REALTIME flag is set in the provided
+ structure.
-5. Invoke the KVM_GET_CLOCK ioctl to record the host TSC (t_1) and
- kvmclock nanoseconds (k_1).
+ KVM will advance the VM's kvmclock to account for elapsed time since
+ recording the clock values. Note that this will cause problems in
+ the guest (e.g., timeouts) unless CLOCK_REALTIME is synchronized
+ between the source and destination, and a reasonably short time passes
+ between the source pausing the VMs and the destination executing
+ steps 4-7.
+
+5. Invoke the KVM_GET_CLOCK ioctl to record the host TSC (tsc_dest) and
+ kvmclock nanoseconds (guest_dest).
6. Adjust the guest TSC offsets for every vCPU to account for (1) time
elapsed since recording state and (2) difference in TSCs between the
source and destination machine:
- new_off_n = t_0 + off_n + (k_1 - k_0) * freq - t_1
+ ofs_dst[i] = ofs_src[i] -
+ (guest_src - guest_dest) * freq +
+ (tsc_src - tsc_dest)
+
+ ("ofs[i] + tsc - guest * freq" is the guest TSC value corresponding to
+ a time of 0 in kvmclock. The above formula ensures that it is the
+ same on the destination as it was on the source).
7. Write the KVM_VCPU_TSC_OFFSET attribute for every vCPU with the
respective value derived in the previous step.
--- /dev/null
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2021, Google LLC.
+ *
+ * Tests for adjusting the system counter from userspace
+ */
+#include <asm/kvm_para.h>
+#include <stdint.h>
+#include <string.h>
+#include <sys/stat.h>
+#include <time.h>
+
+#include "test_util.h"
+#include "kvm_util.h"
+#include "processor.h"
+
+#define VCPU_ID 0
+
+#ifdef __x86_64__
+
+struct test_case {
+ uint64_t tsc_offset;
+};
+
+static struct test_case test_cases[] = {
+ { 0 },
+ { 180 * NSEC_PER_SEC },
+ { -180 * NSEC_PER_SEC },
+};
+
+static void check_preconditions(struct kvm_vm *vm)
+{
+ if (!_vcpu_has_device_attr(vm, VCPU_ID, KVM_VCPU_TSC_CTRL, KVM_VCPU_TSC_OFFSET))
+ return;
+
+ print_skip("KVM_VCPU_TSC_OFFSET not supported; skipping test");
+ exit(KSFT_SKIP);
+}
+
+static void setup_system_counter(struct kvm_vm *vm, struct test_case *test)
+{
+ vcpu_access_device_attr(vm, VCPU_ID, KVM_VCPU_TSC_CTRL,
+ KVM_VCPU_TSC_OFFSET, &test->tsc_offset, true);
+}
+
+static uint64_t guest_read_system_counter(struct test_case *test)
+{
+ return rdtsc();
+}
+
+static uint64_t host_read_guest_system_counter(struct test_case *test)
+{
+ return rdtsc() + test->tsc_offset;
+}
+
+#else /* __x86_64__ */
+
+#error test not implemented for this architecture!
+
+#endif
+
+#define GUEST_SYNC_CLOCK(__stage, __val) \
+ GUEST_SYNC_ARGS(__stage, __val, 0, 0, 0)
+
+static void guest_main(void)
+{
+ int i;
+
+ for (i = 0; i < ARRAY_SIZE(test_cases); i++) {
+ struct test_case *test = &test_cases[i];
+
+ GUEST_SYNC_CLOCK(i, guest_read_system_counter(test));
+ }
+}
+
+static void handle_sync(struct ucall *uc, uint64_t start, uint64_t end)
+{
+ uint64_t obs = uc->args[2];
+
+ TEST_ASSERT(start <= obs && obs <= end,
+ "unexpected system counter value: %"PRIu64" expected range: [%"PRIu64", %"PRIu64"]",
+ obs, start, end);
+
+ pr_info("system counter value: %"PRIu64" expected range [%"PRIu64", %"PRIu64"]\n",
+ obs, start, end);
+}
+
+static void handle_abort(struct ucall *uc)
+{
+ TEST_FAIL("%s at %s:%ld", (const char *)uc->args[0],
+ __FILE__, uc->args[1]);
+}
+
+static void enter_guest(struct kvm_vm *vm)
+{
+ uint64_t start, end;
+ struct ucall uc;
+ int i;
+
+ for (i = 0; i < ARRAY_SIZE(test_cases); i++) {
+ struct test_case *test = &test_cases[i];
+
+ setup_system_counter(vm, test);
+ start = host_read_guest_system_counter(test);
+ vcpu_run(vm, VCPU_ID);
+ end = host_read_guest_system_counter(test);
+
+ switch (get_ucall(vm, VCPU_ID, &uc)) {
+ case UCALL_SYNC:
+ handle_sync(&uc, start, end);
+ break;
+ case UCALL_ABORT:
+ handle_abort(&uc);
+ return;
+ default:
+ TEST_ASSERT(0, "unhandled ucall %ld\n",
+ get_ucall(vm, VCPU_ID, &uc));
+ }
+ }
+}
+
+int main(void)
+{
+ struct kvm_vm *vm;
+
+ vm = vm_create_default(VCPU_ID, 0, guest_main);
+ check_preconditions(vm);
+ ucall_init(vm, NULL);
+
+ enter_guest(vm);
+ kvm_vm_free(vm);
+}
projects / platform / kernel / linux-rpi.git / commitdiff