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@xxxxxxxxxx>
> >
> > ---
> > 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
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