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

Pankaj Gupta
2016-12-13T01:32:18Z

>
> >
> > On Mon, Dec 12, 2016 at 02:36:55AM -0500, Pankaj Gupta wrote:
> > >
> > > Hello Marcelo,
> >
> > Hi Pankaj,
> >
> > > > 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'?
> >
> > Yes it can happen: migration overwrites s->clock_is_reliable value (but
> > you can't see that from grepping).
>
> o.k.

Reviewed-by: Pankaj Gupta <pagupta@redhat.com>

> >
> > > > * 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);
> > > >
> > > >
> > > >
> > > >
> >
> >
> --
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