Re: [PATCH v2 1/2] KVM: x86: Add KVM_[GS]ET_CLOCK_GUEST for accurate KVM clock migration

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On 10/04/2024 10:52, Jack Allister wrote:
In the common case (where kvm->arch.use_master_clock is true), the KVM
clock is defined as a simple arithmetic function of the guest TSC, based on
a reference point stored in kvm->arch.master_kernel_ns and

The existing KVM_[GS]ET_CLOCK functionality does not allow for this
relationship to be precisely saved and restored by userspace. All it can
currently do is set the KVM clock at a given UTC reference time, which is
necessarily imprecise.

So on live update, the guest TSC can remain cycle accurate at precisely the
same offset from the host TSC, but there is no way for userspace to restore
the KVM clock accurately.

Even on live migration to a new host, where the accuracy of the guest time-
keeping is fundamentally limited by the accuracy of wallclock
synchronization between the source and destination hosts, the clock jump
experienced by the guest's TSC and its KVM clock should at least be
*consistent*. Even when the guest TSC suffers a discontinuity, its KVM
clock should still remain the *same* arithmetic function of the guest TSC,
and not suffer an *additional* discontinuity.

To allow for accurate migration of the KVM clock, add per-vCPU ioctls which
save and restore the actual PV clock info in pvclock_vcpu_time_info.

The restoration in KVM_SET_CLOCK_GUEST works by creating a new reference
point in time just as kvm_update_masterclock() does, and calculating the
corresponding guest TSC value. This guest TSC value is then passed through
the user-provided pvclock structure to generate the *intended* KVM clock
value at that point in time, and through the *actual* KVM clock calculation.
Then kvm->arch.kvmclock_offset is adjusted to eliminate for the difference.

Where kvm->arch.use_master_clock is false (because the host TSC is
unreliable, or the guest TSCs are configured strangely), the KVM clock
is *not* defined as a function of the guest TSC so KVM_GET_CLOCK_GUEST
returns an error. In this case, as documented, userspace shall use the
legacy KVM_GET_CLOCK ioctl. The loss of precision is acceptable in this
case since the clocks are imprecise in this mode anyway.

On *restoration*, if kvm->arch.use_master_clock is false, an error is
returned for similar reasons and userspace shall fall back to using
KVM_SET_CLOCK. This does mean that, as documented, userspace needs to use
*both* KVM_GET_CLOCK_GUEST and KVM_GET_CLOCK and send both results with the
migration data (unless the intent is to refuse to resume on a host with bad

(It may have been possible to make KVM_SET_CLOCK_GUEST "good enough" in the
non-masterclock mode, as that mode is necessarily imprecise anyway. The
explicit fallback allows userspace to deliberately fail migration to a host
with misbehaving TSC where master clock mode wouldn't be active.)

Suggested-by: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
Signed-off-by: Jack Allister <jalliste@xxxxxxxxxx>
CC: Paul Durrant <paul@xxxxxxx>
CC: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
  Documentation/virt/kvm/api.rst |  37 ++++++++++
  arch/x86/kvm/x86.c             | 124 +++++++++++++++++++++++++++++++++
  include/uapi/linux/kvm.h       |   3 +
  3 files changed, 164 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 0b5a33ee71ee..80fcd93bba1b 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6352,6 +6352,43 @@ a single guest_memfd file, but the bound ranges must not overlap).
See KVM_SET_USER_MEMORY_REGION2 for additional details. +4.143 KVM_GET_CLOCK_GUEST
+:Capability: none
+:Architectures: x86
+:Type: vcpu ioctl
+:Parameters: struct pvclock_vcpu_time_info (out)
+:Returns: 0 on success, <0 on error
+Retrieves the current time information structure used for KVM/PV clocks,
+in precisely the form advertised to the guest vCPU, which gives parameters
+for a direct conversion from a guest TSC value to nanoseconds.
+When the KVM clock not is in "master clock" mode, for example because the
+host TSC is unreliable or the guest TSCs are oddly configured, the KVM clock
+is actually defined by the host CLOCK_MONOTONIC_RAW instead of the guest TSC.
+In this case, the KVM_GET_CLOCK_GUEST ioctl returns -EINVAL.
+:Capability: none
+:Architectures: x86
+:Type: vcpu ioctl
+:Parameters: struct pvclock_vcpu_time_info (in)
+:Returns: 0 on success, <0 on error
+Sets the KVM clock (for the whole VM) in terms of the vCPU TSC, using the
+pvclock structure as returned by KVM_GET_CLOCK_GUEST. This allows the precise
+arithmetic relationship between guest TSC and KVM clock to be preserved by
+userspace across migration.
+When the KVM clock is not in "master clock" mode, and the KVM clock is actually
+defined by the host CLOCK_MONOTONIC_RAW, this ioctl returns -EINVAL.

EINVAL doesn't seem appropriate. ENOTSUP perhaps? Same for getting the clock info I suppose.

+may choose to set the clock using the less precise KVM_SET_CLOCK ioctl, or may
+choose to fail, denying migration to a host whose TSC is misbehaving.
  5. The kvm_run structure
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 47d9f03b7778..d5cae3ead04d 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5859,6 +5859,124 @@ static int kvm_vcpu_ioctl_enable_cap(struct kvm_vcpu *vcpu,
+static int kvm_vcpu_ioctl_get_clock_guest(struct kvm_vcpu *v, void __user *argp)
+	struct pvclock_vcpu_time_info *vcpu_pvti = &v->arch.hv_clock;
+	struct pvclock_vcpu_time_info local_pvti = { 0 };
+	struct kvm_arch *ka = &v->kvm->arch;
+	uint64_t host_tsc, guest_tsc;
+	bool use_master_clock;
+	uint64_t kernel_ns;
+	unsigned int seq;
+	/*
+	 * CLOCK_MONOTONIC_RAW is not suitable for GET/SET API,
+	 * see kvm_vcpu_ioctl_set_clock_guest equivalent comment.
+	 */
+	if (!static_cpu_has(X86_FEATURE_CONSTANT_TSC))
+		return -EINVAL;
+	/*
+	 * Querying needs to be performed in a seqcount loop as it's possible
+	 * another vCPU has triggered an update of the master clock. If so we
+	 * should store the host TSC & time at this point.
+	 */
+	do {
+		seq = read_seqcount_begin(&ka->pvclock_sc);
+		use_master_clock = ka->use_master_clock;
+		if (use_master_clock) {
+			host_tsc = ka->master_cycle_now;
+			kernel_ns = ka->master_kernel_ns;
+		}
+	} while (read_seqcount_retry(&ka->pvclock_sc, seq));

You could bail from the loop if `use_master_clock` is false, couldn't you?

+	if (!use_master_clock)
+		return -EINVAL;
+	/*
+	 * It's possible that this vCPU doesn't have a HVCLOCK configured
+	 * but the other vCPUs may. If this is the case calculate based
+	 * upon the time gathered in the seqcount but do not update the
+	 * vCPU specific PVTI. If we have one, then use that.

Given this is a per-vCPU ioctl, why not fail in the case the vCPU doesn't have HVCLOCK configured? Or is your intention that a GET/SET should always work if TSC is stable?

+	 */
+	if (!vcpu_pvti->tsc_timestamp && !vcpu_pvti->system_time) {
+		guest_tsc = kvm_read_l1_tsc(v, host_tsc);
+		local_pvti.tsc_timestamp = guest_tsc;
+		local_pvti.system_time = kernel_ns + ka->kvmclock_offset;
+	} else {
+		local_pvti = *vcpu_pvti;
+	}
+	if (copy_to_user(argp, &local_pvti, sizeof(local_pvti)))
+		return -EFAULT;
+	return 0;

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