Re: [Qemu-devel] [qemu patch V4 2/2] kvmclock: reduce kvmclock difference on migration

[Pankaj Gupta <pagupta@redhat.com>]

Hello Marcelo,

> 
> Check for KVM_CAP_ADJUST_CLOCK capability KVM_CLOCK_TSC_STABLE, which
> indicates that KVM_GET_CLOCK returns a value as seen by the guest at
> that moment.
> 
> For new machine types, use this value rather than reading
> from guest memory.
> 
> This reduces kvmclock difference on migration from 5s to 0.1s
> (when max_downtime == 5s).
> 
> Note: pre_save contradicts the following comment
>         /*
>          * If the VM is stopped, declare the clock state valid to
>          * avoid re-reading it on next vmsave (which would return
>          * a different value). Will be reset when the VM is continued.
>          */
> But the comment is bogus: vm_state_change is never called twice in a row
> with running=0 or running=1.
> 
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> 
> ---
>  hw/i386/kvm/clock.c    |  107
>  ++++++++++++++++++++++++++++++++++++++++++-------
>  include/hw/i386/pc.h   |    5 ++
>  target-i386/kvm.c      |    7 +++
>  target-i386/kvm_i386.h |    1
>  4 files changed, 106 insertions(+), 14 deletions(-)
> 
> v2:
> - improve variable names (Juan)
> - consolidate code on kvm_get_clock function (Paolo)
> - return mach_use_reliable_get_clock from needed function (Paolo)
> v3:
> - simplify check for src_use_reliable_get_clock (Eduardo)
> - move src_use_reliable_get_clock initialization to realize (Eduardo)
> 
> v4:
> - have kvm_get_clock and clock_is_reliable assignments synchronized (Eduardo)
> - add comment to the reasoning of kvmclock_pre_save (Eduardo)
> 
> 
> Index: qemu-mig-advance-clock/hw/i386/kvm/clock.c
> ===================================================================
> --- qemu-mig-advance-clock.orig/hw/i386/kvm/clock.c	2016-11-17
> 15:07:11.220632761 -0200
> +++ qemu-mig-advance-clock/hw/i386/kvm/clock.c	2016-12-10 13:57:58.115983966
> -0200
> @@ -36,6 +36,13 @@
>  
>      uint64_t clock;
>      bool clock_valid;
> +
> +    /* whether machine type supports reliable get clock */
> +    bool mach_use_reliable_get_clock;
> +
> +    /* whether the 'clock' value was obtained in a host with
> +     * reliable KVM_GET_CLOCK */
> +    bool clock_is_reliable;
>  } KVMClockState;
>  
>  struct pvclock_vcpu_time_info {
> @@ -81,6 +88,19 @@
>      return nsec + time.system_time;
>  }
>  
> +static uint64_t kvm_get_clock(void)
> +{
> +    struct kvm_clock_data data;
> +    int ret;
> +
> +    ret = kvm_vm_ioctl(kvm_state, KVM_GET_CLOCK, &data);
> +    if (ret < 0) {
> +        fprintf(stderr, "KVM_GET_CLOCK failed: %s\n", strerror(ret));
> +                abort();
> +    }
> +    return data.clock;
> +}
> +
>  static void kvmclock_vm_state_change(void *opaque, int running,
>                                       RunState state)
>  {
> @@ -91,15 +111,23 @@
>  
>      if (running) {
>          struct kvm_clock_data data = {};
> -        uint64_t time_at_migration = kvmclock_current_nsec(s);
> +        uint64_t pvclock_via_mem = 0;
>  
> -        s->clock_valid = false;
> +        /*
> +         * If the host where s->clock was read did not support reliable
> +         * KVM_GET_CLOCK, read kvmclock value from memory.
> +         */
> +        if (!s->clock_is_reliable) {
> +            pvclock_via_mem = kvmclock_current_nsec(s);
> +        }
>  
> -        /* We can't rely on the migrated clock value, just discard it */
> -        if (time_at_migration) {
> -            s->clock = time_at_migration;
> +        /* We can't rely on the saved clock value, just discard it */
> +        if (pvclock_via_mem) {
> +            s->clock = pvclock_via_mem;
>          }
>  
> +        s->clock_valid = false;
> +
>          data.clock = s->clock;
>          ret = kvm_vm_ioctl(kvm_state, KVM_SET_CLOCK, &data);
>          if (ret < 0) {
> @@ -120,8 +148,6 @@
>              }
>          }
>      } else {
> -        struct kvm_clock_data data;
> -        int ret;
>  
>          if (s->clock_valid) {
>              return;
> @@ -129,13 +155,11 @@
>  
>          kvm_synchronize_all_tsc();
>  
> -        ret = kvm_vm_ioctl(kvm_state, KVM_GET_CLOCK, &data);
> -        if (ret < 0) {
> -            fprintf(stderr, "KVM_GET_CLOCK failed: %s\n", strerror(ret));
> -            abort();
> -        }
> -        s->clock = data.clock;
> -
> +        s->clock = kvm_get_clock();
> +        /* any code that sets s->clock needs to ensure clock_is_reliable
> +         * is correctly set.
> +         */
> +        s->clock_is_reliable = kvm_has_adjust_clock_stable();
          
Is it required to call  'kvm_has_adjust_clock_stable' here. Is there any chance
of getting different value then in 'realize'? 

>          /*
>           * If the VM is stopped, declare the clock state valid to
>           * avoid re-reading it on next vmsave (which would return
> @@ -149,25 +173,80 @@
>  {
>      KVMClockState *s = KVM_CLOCK(dev);
>  
> +    if (kvm_has_adjust_clock_stable()) {
> +        s->clock_is_reliable = true;
> +    }
> +
>      qemu_add_vm_change_state_handler(kvmclock_vm_state_change, s);
>  }
>  
> +static bool kvmclock_clock_is_reliable_needed(void *opaque)
> +{
> +    KVMClockState *s = opaque;
> +
> +    return s->mach_use_reliable_get_clock;
> +}
> +
> +static const VMStateDescription kvmclock_reliable_get_clock = {
> +    .name = "kvmclock/clock_is_reliable",
> +    .version_id = 1,
> +    .minimum_version_id = 1,
> +    .needed = kvmclock_clock_is_reliable_needed,
> +    .fields = (VMStateField[]) {
> +        VMSTATE_BOOL(clock_is_reliable, KVMClockState),
> +        VMSTATE_END_OF_LIST()
> +    }
> +};
> +
> +/*
> + * When migrating, read the clock just before migration,
> + * so that the guest clock counts during the events
> + * between:
> + *
> + *  * vm_stop()
> + *  *
> + *  * pre_save()
> + *
> + *  This reduces kvmclock difference on migration from 5s
> + *  to 0.1s (when max_downtime == 5s), because sending the
> + *  final pages of memory (which happens between vm_stop()
> + *  and pre_save()) takes max_downtime.
> + */
> +static void kvmclock_pre_save(void *opaque)
> +{
> +    KVMClockState *s = opaque;
> +
> +    s->clock = kvm_get_clock();
> +}
> +
>  static const VMStateDescription kvmclock_vmsd = {
>      .name = "kvmclock",
>      .version_id = 1,
>      .minimum_version_id = 1,
> +    .pre_save = kvmclock_pre_save,
>      .fields = (VMStateField[]) {
>          VMSTATE_UINT64(clock, KVMClockState),
>          VMSTATE_END_OF_LIST()
> +    },
> +    .subsections = (const VMStateDescription * []) {
> +        &kvmclock_reliable_get_clock,
> +        NULL
>      }
>  };
>  
> +static Property kvmclock_properties[] = {
> +    DEFINE_PROP_BOOL("x-mach-use-reliable-get-clock", KVMClockState,
> +                      mach_use_reliable_get_clock, true),
> +    DEFINE_PROP_END_OF_LIST(),
> +};
> +
>  static void kvmclock_class_init(ObjectClass *klass, void *data)
>  {
>      DeviceClass *dc = DEVICE_CLASS(klass);
>  
>      dc->realize = kvmclock_realize;
>      dc->vmsd = &kvmclock_vmsd;
> +    dc->props = kvmclock_properties;
>  }
>  
>  static const TypeInfo kvmclock_info = {
> Index: qemu-mig-advance-clock/include/hw/i386/pc.h
> ===================================================================
> --- qemu-mig-advance-clock.orig/include/hw/i386/pc.h	2016-11-17
> 15:07:11.220632761 -0200
> +++ qemu-mig-advance-clock/include/hw/i386/pc.h	2016-11-17 15:08:58.096791416
> -0200
> @@ -370,6 +370,11 @@
>  #define PC_COMPAT_2_7 \
>      HW_COMPAT_2_7 \
>      {\
> +        .driver   = "kvmclock",\
> +        .property = "x-mach-use-reliable-get-clock",\
> +        .value    = "off",\
> +    },\
> +    {\
>          .driver   = TYPE_X86_CPU,\
>          .property = "l3-cache",\
>          .value    = "off",\
> Index: qemu-mig-advance-clock/target-i386/kvm.c
> ===================================================================
> --- qemu-mig-advance-clock.orig/target-i386/kvm.c	2016-11-17
> 15:07:11.221632762 -0200
> +++ qemu-mig-advance-clock/target-i386/kvm.c	2016-11-17 15:07:15.867639659
> -0200
> @@ -117,6 +117,13 @@
>      return kvm_check_extension(kvm_state, KVM_CAP_X86_SMM);
>  }
>  
> +bool kvm_has_adjust_clock_stable(void)
> +{
> +    int ret = kvm_check_extension(kvm_state, KVM_CAP_ADJUST_CLOCK);
> +
> +    return (ret == KVM_CLOCK_TSC_STABLE);
> +}
> +
>  bool kvm_allows_irq0_override(void)
>  {
>      return !kvm_irqchip_in_kernel() || kvm_has_gsi_routing();
> Index: qemu-mig-advance-clock/target-i386/kvm_i386.h
> ===================================================================
> --- qemu-mig-advance-clock.orig/target-i386/kvm_i386.h	2016-11-17
> 15:07:11.222632764 -0200
> +++ qemu-mig-advance-clock/target-i386/kvm_i386.h	2016-11-17
> 15:07:15.867639659 -0200
> @@ -17,6 +17,7 @@
>  
>  bool kvm_allows_irq0_override(void);
>  bool kvm_has_smm(void);
> +bool kvm_has_adjust_clock_stable(void);
>  void kvm_synchronize_all_tsc(void);
>  void kvm_arch_reset_vcpu(X86CPU *cs);
>  void kvm_arch_do_init_vcpu(X86CPU *cs);
> 
> 
> 
>