Re: [qemu patch V3 2/2] kvmclock: reduce kvmclock difference on migration

[Marcelo Tosatti <mtosatti@redhat.com>]

On Mon, Nov 28, 2016 at 12:13:22PM -0200, Eduardo Habkost wrote:
> Sorry for not noticing the following issues on the previous
> reviews. I was only paying attention to the vmstate and machine
> code:
> 
> On Mon, Nov 21, 2016 at 08:50:04AM -0200, Marcelo Tosatti wrote:
> > 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).
> > 
> > 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)
> > 
> > 
> > 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	2016-11-17 15:11:51.372048640 -0200
> > @@ -36,6 +36,12 @@
> >  
> >      uint64_t clock;
> >      bool clock_valid;
> > +
> > +    /* whether machine supports reliable KVM_GET_CLOCK */
> > +    bool mach_use_reliable_get_clock;
> > +
> > +    /* whether source host supported reliable KVM_GET_CLOCK */
> > +    bool src_use_reliable_get_clock;
> >  } KVMClockState;
> >  
> >  struct pvclock_vcpu_time_info {
> > @@ -81,6 +87,19 @@
> >      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 +110,36 @@
> 
> Can you please use "diff -p" on your patches?
> 
> >  
> >      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;
> > +        /* local (running VM) restore */
> > +        if (s->clock_valid) {
> > +            /*
> > +             * if host does not support reliable KVM_GET_CLOCK,
> > +             * read kvmclock value from memory
> > +             */
> > +            if (!kvm_has_adjust_clock_stable()) {
> 
> Why not use src_use_reliable_get_clock here, too? (Maybe rename
> it to simply clock_is_reliable?)

Because you end up mixing two mental ideas (one: whether the source
has KVM_GET_CLOCK, second: whether the destination has KVM_GET_CLOCK 
into one variable). I find it more confusing than not.
Maybe its just my limited brain but i find it
confusing.

> You can change kvm_get_clock() to kvm_get_clock(KVMClockState*),
> and make it update both s->clock and s->clock_is_reliable. With
> this, s->clock and s->clock_is_reliable will be always in sync,
> and have a single code path for both local restore and migration:

There should not be a single path for migration/runtime cases: 
on migration, we care whether the source used reliable KVM_GET_CLOCK.

On runtime, we care whether the destination uses reliable KVM_GET_CLOCK.

>     /*
>      * If host that set s->clock did not support reliable
>      * KVM_GET_CLOCK, read kvmclock value from memory
>      */
>     if (!s->clock_is_reliable) {
>         pvclock_via_mem = kvmclock_current_nsec(s);
>     }
> 
> Updating the table from my previous message, the result would be:
> 
> --------+------------+---------------+-----------+---------+
>         | kvm_has_adj_clock_stable() | clock_is_reliable   |
> machine | src        | dst           | src       | dst (*) |
> --------+------------+---------------+-----------+---------+
> pc-2.8  | false      | false         | false     | false   |
> pc-2.8  | false      | true          | false     | false   |
> pc-2.8  | true       | false         | true      | true    |
> pc-2.8  | true       | true          | true      | true    |
> --------+------------+---------------+-----------+---------+
> pc-2.7  | false      | (any)         | false     | false   |
> pc-2.7  | true       | (any)         | true (**) | false   |
> --------+------------+---------------+-----------+---------+
> 
> (*) More precisely: in the first kvmclock_vm_state_change() call
>     just after migration. It will get updated the next time
>     s->clock is set.
> 
> (**) On purpose, so the code I suggest above would work for local
>      restore, too.
> 
> 
> > +                pvclock_via_mem = kvmclock_current_nsec(s);
> > +            }
> > +        /* migration/savevm/init restore */
> > +        } else {
> > +            /*
> > +             * use s->clock in case machine uses reliable
> > +             * get clock and source host supported
> > +             * reliable get clock
> > +             */
> > +            if (!s->src_use_reliable_get_clock) {
> > +                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 +160,6 @@
> >              }
> >          }
> >      } else {
> > -        struct kvm_clock_data data;
> > -        int ret;
> >  
> >          if (s->clock_valid) {
> >              return;
> > @@ -129,13 +167,7 @@
> >  
> >          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();
> >          /*
> >           * If the VM is stopped, declare the clock state valid to
> >           * avoid re-reading it on next vmsave (which would return
> > @@ -149,25 +181,72 @@
> >  {
> >      KVMClockState *s = KVM_CLOCK(dev);
> >  
> > +    /*
> > +     * On machine types that support reliable KVM_GET_CLOCK,
> > +     * if host kernel does provide reliable KVM_GET_CLOCK,
> > +     * set src_use_reliable_get_clock=true so that destination
> > +     * avoids reading kvmclock from memory.
> > +     */
> > +    if (s->mach_use_reliable_get_clock && kvm_has_adjust_clock_stable()) {
> > +        s->src_use_reliable_get_clock = true;
> > +    }
> > +
> >      qemu_add_vm_change_state_handler(kvmclock_vm_state_change, s);
> >  }
> >  
> > +static bool kvmclock_src_use_reliable_get_clock(void *opaque)
> > +{
> > +    KVMClockState *s = opaque;
> > +
> > +    return s->mach_use_reliable_get_clock;
> > +}
> > +
> > +static const VMStateDescription kvmclock_reliable_get_clock = {
> > +    .name = "kvmclock/src_use_reliable_get_clock",
> > +    .version_id = 1,
> > +    .minimum_version_id = 1,
> > +    .needed = kvmclock_src_use_reliable_get_clock,
> > +    .fields = (VMStateField[]) {
> > +        VMSTATE_BOOL(src_use_reliable_get_clock, KVMClockState),
> > +        VMSTATE_END_OF_LIST()
> > +    }
> > +};
> > +
> > +static void kvmclock_pre_save(void *opaque)
> > +{
> > +    KVMClockState *s = opaque;
> > +
> > +    s->clock = kvm_get_clock();
> 
> I can't tell if s->clock_valid here is a bug, or a feature. In
> either case, we seem to be contradicting the comment at
> kvmclock_vm_state_change():
> 
>     /*
>      * 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.
>      */
>     s->clock_valid = true;

Sorry, what is the problem??

> 
> > +}
> > +
> >  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);
> > 
> > 
> 
> -- 
> Eduardo