Re: [PATCH 03/10] KVM: x86: Add KVM_[GS]ET_CLOCK_GUEST for accurate KVM clock migration

[Paolo Bonzini]

On Thu, Apr 18, 2024 at 9:46 PM David Woodhouse <dwmw2@xxxxxxxxxxxxx> wrote:
> +       curr_tsc_hz = get_cpu_tsc_khz() * 1000LL;
> +       if (unlikely(curr_tsc_hz == 0)) {
> +               rc = -EINVAL;
> +               goto out;
> +       }
> +
> +       if (kvm_caps.has_tsc_control)
> +               curr_tsc_hz = kvm_scale_tsc(curr_tsc_hz,
> +                                           v->arch.l1_tsc_scaling_ratio);
> +
> +       /*
> +        * The scaling factors in the hv_clock do not depend solely on the
> +        * TSC frequency *requested* by userspace. They actually use the
> +        * host TSC frequency that was measured/detected by the host kernel,
> +        * scaled by kvm_scale_tsc() with the vCPU's l1_tsc_scaling_ratio.
> +        * So a sanity check that they *precisely* match would have false
> +        * negatives. Allow for a discrepancy of 1 kHz either way.

This is not very clear - if kvm_caps.has_tsc_control, cur_tsc_hz is
exactly the "host TSC frequency [...] scaled by kvm_scale_tsc() with
the vCPU's l1_tsc_scaling_ratio". But even in that case there is a
double rounding issue, I guess.

> +       /*
> +        * The call to pvclock_update_vm_gtod_copy() has created a new time
> +        * reference point in ka->master_cycle_now and ka->master_kernel_ns.
> +        *
> +        * Calculate the guest TSC at that moment, and the corresponding KVM
> +        * clock value according to user_hv_clock. The value according to the
> +        * current hv_clock will of course be ka->master_kernel_ns since no
> +        * TSC cycles have elapsed.
> +        *
> +        * Adjust ka->kvmclock_offset to the delta, so that both definitions
> +        * of the clock give precisely the same reading at the reference time.
> +        */
> +       guest_tsc = kvm_read_l1_tsc(v, ka->master_cycle_now);
> +       user_clk_ns = __pvclock_read_cycles(&user_hv_clock, guest_tsc);
> +       ka->kvmclock_offset = user_clk_ns - ka->master_kernel_ns;
> +
> +out:
> +       kvm_end_pvclock_update(kvm);
> +       return rc;
> +}
> +#endif
> +
>  long kvm_arch_vcpu_ioctl(struct file *filp,
>                          unsigned int ioctl, unsigned long arg)
>  {
> @@ -6256,6 +6404,14 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
>                 srcu_read_unlock(&vcpu->kvm->srcu, idx);
>                 break;
>         }
> +#ifdef CONFIG_X86_64
> +       case KVM_SET_CLOCK_GUEST:
> +               r = kvm_vcpu_ioctl_set_clock_guest(vcpu, argp);
> +               break;
> +       case KVM_GET_CLOCK_GUEST:
> +               r = kvm_vcpu_ioctl_get_clock_guest(vcpu, argp);
> +               break;
> +#endif
>  #ifdef CONFIG_KVM_HYPERV
>         case KVM_GET_SUPPORTED_HV_CPUID:
>                 r = kvm_ioctl_get_supported_hv_cpuid(vcpu, argp);
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 2190adbe3002..0d306311e4d6 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1548,4 +1548,7 @@ struct kvm_create_guest_memfd {
>         __u64 reserved[6];
>  };
>
> +#define KVM_SET_CLOCK_GUEST       _IOW(KVMIO,  0xd5, struct pvclock_vcpu_time_info)
> +#define KVM_GET_CLOCK_GUEST       _IOR(KVMIO,  0xd6, struct pvclock_vcpu_time_info)
> +
>  #endif /* __LINUX_KVM_H */
> --
> 2.44.0
>

On Thu, Apr 18, 2024 at 9:46 PM David Woodhouse <dwmw2@xxxxxxxxxxxxx> wrote:
>
> From: Jack Allister <jalliste@xxxxxxxxxx>
>
> 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
> kvm->arch.master_cycle_now.
>
> 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
> TSC).
>
> (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.)
>
> Co-developed-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
> 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>
> ---
>  Documentation/virt/kvm/api.rst |  37 ++++++++
>  arch/x86/kvm/x86.c             | 156 +++++++++++++++++++++++++++++++++
>  include/uapi/linux/kvm.h       |   3 +
>  3 files changed, 196 insertions(+)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index f0b76ff5030d..758f6fc08fe5 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_64
> +: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.
> +
> +4.144 KVM_SET_CLOCK_GUEST
> +----------------------------
> +
> +:Capability: none
> +:Architectures: x86_64
> +: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. Userspace
> +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 23281c508c27..42abce7b4fc9 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5868,6 +5868,154 @@ static int kvm_vcpu_ioctl_enable_cap(struct kvm_vcpu *vcpu,
>         }
>  }
>
> +#ifdef CONFIG_X86_64
> +static int kvm_vcpu_ioctl_get_clock_guest(struct kvm_vcpu *v, void __user *argp)
> +{
> +       struct pvclock_vcpu_time_info *hv_clock = &v->arch.hv_clock;
> +
> +       /*
> +        * If KVM_REQ_CLOCK_UPDATE is already pending, or if the hv_clock has
> +        * never been generated at all, call kvm_guest_time_update() to do so.
> +        * Might as well use the PVCLOCK_TSC_STABLE_BIT as the check for ever
> +        * having been written.
> +        */
> +       if (kvm_check_request(KVM_REQ_CLOCK_UPDATE, v) ||
> +           !(hv_clock->flags & PVCLOCK_TSC_STABLE_BIT)) {
> +               if (kvm_guest_time_update(v))
> +                       return -EINVAL;
> +       }
> +
> +       /*
> +        * PVCLOCK_TSC_STABLE_BIT is set in use_master_clock mode where the
> +        * KVM clock is defined in terms of the guest TSC. Otherwise, it is
> +        * is defined by the host CLOCK_MONOTONIC_RAW, and userspace should
> +        * use the legacy KVM_[GS]ET_CLOCK to migrate it.
> +        */
> +       if (!(hv_clock->flags & PVCLOCK_TSC_STABLE_BIT))
> +               return -EINVAL;
> +
> +       if (copy_to_user(argp, hv_clock, sizeof(*hv_clock)))
> +               return -EFAULT;
> +
> +       return 0;
> +}
> +
> +/*
> + * Reverse the calculation in the hv_clock definition.
> + *
> + * time_ns = ( (cycles << shift) * mul ) >> 32;
> + * (although shift can be negative, so that's bad C)
> + *
> + * So for a single second,
> + *  NSEC_PER_SEC = ( ( FREQ_HZ << shift) * mul ) >> 32
> + *  NSEC_PER_SEC << 32 = ( FREQ_HZ << shift ) * mul
> + *  ( NSEC_PER_SEC << 32 ) / mul = FREQ_HZ << shift
> + *  ( NSEC_PER_SEC << 32 ) / mul ) >> shift = FREQ_HZ
> + */
> +static uint64_t hvclock_to_hz(uint32_t mul, int8_t shift)
> +{
> +       uint64_t tm = NSEC_PER_SEC << 32;
> +
> +       /* Maximise precision. Shift right until the top bit is set */
> +       tm <<= 2;
> +       shift += 2;
> +
> +       /* While 'mul' is even, increase the shift *after* the division */
> +       while (!(mul & 1)) {
> +               shift++;
> +               mul >>= 1;
> +       }
> +
> +       tm /= mul;
> +
> +       if (shift > 0)
> +               return tm >> shift;
> +       else
> +               return tm << -shift;
> +}
> +
> +static int kvm_vcpu_ioctl_set_clock_guest(struct kvm_vcpu *v, void __user *argp)
> +{
> +       struct pvclock_vcpu_time_info user_hv_clock;
> +       struct kvm *kvm = v->kvm;
> +       struct kvm_arch *ka = &kvm->arch;
> +       uint64_t curr_tsc_hz, user_tsc_hz;
> +       uint64_t user_clk_ns;
> +       uint64_t guest_tsc;
> +       int rc = 0;
> +
> +       if (copy_from_user(&user_hv_clock, argp, sizeof(user_hv_clock)))
> +               return -EFAULT;
> +
> +       if (!user_hv_clock.tsc_to_system_mul)
> +               return -EINVAL;
> +
> +       user_tsc_hz = hvclock_to_hz(user_hv_clock.tsc_to_system_mul,
> +                                   user_hv_clock.tsc_shift);
> +
> +
> +       kvm_hv_request_tsc_page_update(kvm);
> +       kvm_start_pvclock_update(kvm);
> +       pvclock_update_vm_gtod_copy(kvm);
> +
> +       /*
> +        * If not in use_master_clock mode, do not allow userspace to set
> +        * the clock in terms of the guest TSC. Userspace should either
> +        * fail the migration (to a host with suboptimal TSCs), or should
> +        * knowingly restore the KVM clock using KVM_SET_CLOCK instead.
> +        */
> +       if (!ka->use_master_clock) {
> +               rc = -EINVAL;
> +               goto out;
> +       }
> +
> +       curr_tsc_hz = get_cpu_tsc_khz() * 1000LL;
> +       if (unlikely(curr_tsc_hz == 0)) {
> +               rc = -EINVAL;
> +               goto out;
> +       }
> +
> +       if (kvm_caps.has_tsc_control)
> +               curr_tsc_hz = kvm_scale_tsc(curr_tsc_hz,
> +                                           v->arch.l1_tsc_scaling_ratio);
> +
> +       /*
> +        * The scaling factors in the hv_clock do not depend solely on the
> +        * TSC frequency *requested* by userspace. They actually use the
> +        * host TSC frequency that was measured/detected by the host kernel,
> +        * scaled by kvm_scale_tsc() with the vCPU's l1_tsc_scaling_ratio.
> +        *
> +        * So a sanity check that they *precisely* match would have false
> +        * negatives. Allow for a discrepancy of 1 kHz either way.
> +        */
> +       if (user_tsc_hz < curr_tsc_hz - 1000 ||
> +           user_tsc_hz > curr_tsc_hz + 1000) {
> +               rc = -ERANGE;
> +               goto out;
> +       }
> +
> +       /*
> +        * The call to pvclock_update_vm_gtod_copy() has created a new time
> +        * reference point in ka->master_cycle_now and ka->master_kernel_ns.
> +        *
> +        * Calculate the guest TSC at that moment, and the corresponding KVM
> +        * clock value according to user_hv_clock. The value according to the
> +        * current hv_clock will of course be ka->master_kernel_ns since no
> +        * TSC cycles have elapsed.
> +        *
> +        * Adjust ka->kvmclock_offset to the delta, so that both definitions
> +        * of the clock give precisely the same reading at the reference time.
> +        */
> +       guest_tsc = kvm_read_l1_tsc(v, ka->master_cycle_now);
> +       user_clk_ns = __pvclock_read_cycles(&user_hv_clock, guest_tsc);
> +       ka->kvmclock_offset = user_clk_ns - ka->master_kernel_ns;
> +
> +out:
> +       kvm_end_pvclock_update(kvm);
> +       return rc;
> +}
> +#endif
> +
>  long kvm_arch_vcpu_ioctl(struct file *filp,
>                          unsigned int ioctl, unsigned long arg)
>  {
> @@ -6256,6 +6404,14 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
>                 srcu_read_unlock(&vcpu->kvm->srcu, idx);
>                 break;
>         }
> +#ifdef CONFIG_X86_64
> +       case KVM_SET_CLOCK_GUEST:
> +               r = kvm_vcpu_ioctl_set_clock_guest(vcpu, argp);
> +               break;
> +       case KVM_GET_CLOCK_GUEST:
> +               r = kvm_vcpu_ioctl_get_clock_guest(vcpu, argp);
> +               break;
> +#endif
>  #ifdef CONFIG_KVM_HYPERV
>         case KVM_GET_SUPPORTED_HV_CPUID:
>                 r = kvm_ioctl_get_supported_hv_cpuid(vcpu, argp);
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 2190adbe3002..0d306311e4d6 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1548,4 +1548,7 @@ struct kvm_create_guest_memfd {
>         __u64 reserved[6];
>  };
>
> +#define KVM_SET_CLOCK_GUEST       _IOW(KVMIO,  0xd5, struct pvclock_vcpu_time_info)
> +#define KVM_GET_CLOCK_GUEST       _IOR(KVMIO,  0xd6, struct pvclock_vcpu_time_info)
> +
>  #endif /* __LINUX_KVM_H */
> --
> 2.44.0
>