On Thu, Nov 17, 2016 at 04:16:09PM -0200, Eduardo Habkost wrote:
> On Thu, Nov 17, 2016 at 03:15:35PM -0200, Marcelo Tosatti wrote:
> > On Thu, Nov 17, 2016 at 12:11:26PM -0200, Eduardo Habkost wrote:
> > > On Thu, Nov 17, 2016 at 11:24:13AM -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 | 108 ++++++++++++++++++++++++++++++++++++++++++-------
> > > > include/hw/i386/pc.h | 5 ++
> > > > target-i386/kvm.c | 7 +++
> > > > target-i386/kvm_i386.h | 1
> > > > 4 files changed, 107 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)
> > > >
> > > > Index: qemu-mig-advance-clock/hw/i386/kvm/clock.c
> > > > ===================================================================
> > > > --- qemu-mig-advance-clock.orig/hw/i386/kvm/clock.c 2016-11-14 10:40:39.748116312 -0200
> > > > +++ qemu-mig-advance-clock/hw/i386/kvm/clock.c 2016-11-14 13:38:29.299955042 -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 +110,37 @@
> > > >
> > > > 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()) {
> > > > + 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->mach_use_reliable_get_clock &&
> > > > + s->src_use_reliable_get_clock)) {
> > > > + pvclock_via_mem = kvmclock_current_nsec(s);
> > > > + }
> > >
> > > The s->mach_use_reliable_get_clock check seems redundant.
> > > src_use_reliable_get_clock is set only if
> > > mach_use_reliable_get_clock is true.
> >
> > Done.
> >
> > > > + }
> > > >
> > > > - /* 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 +161,6 @@
> > > > }
> > > > }
> > > > } else {
> > > > - struct kvm_clock_data data;
> > > > - int ret;
> > > >
> > > > if (s->clock_valid) {
> > > > return;
> > > > @@ -129,13 +168,7 @@
> > > >
> > > > 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
> > > > @@ -152,22 +185,69 @@
> > > > qemu_add_vm_change_state_handler(kvmclock_vm_state_change, s);
> > > > }
> > > >
> > > > +static bool kvmclock_src_use_reliable_get_clock(void *opaque)
> > > > +{
> > > > + KVMClockState *s = opaque;
> > > > +
> > > > + /*
> > > > + * 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;
> > > > + }
> > >
> > > It feels fragile to change device state inside the .needed
> > > function. Better to initialize src_use_reliable_get_clock on
> > > kvmclock_realize()?
> > >
> > > What exactly ensures src_use_reliable_get_clock is correctly
> > > initialized on the migration destination as well?
> >
> > Its initialized to false (because its part of device state).
>
> Do you mean it is always going to be false on the destination
> host? What if .needed is called by other code? .needed shouldn't
> have any side-effects.
I don't think needed is called by any other code.
> Also, the difference between the variables on the destination and
> the source is very confusing, see other comments below:
Yes it is: its because you don't know when is what. But if you consider
separate events:
initialization, incoming migration, vmstop, vmcontinue.
And the values of the variables for each case, then it becomes
clear.
If anyone has a suggestion to improve clarify, its welcome.
> > >
> > > > +
> > > > + return s->mach_use_reliable_get_clock;
> > >
> > > If if kvm_has_adjust_clock_stable() is false, isn't it simpler to
> > > simply skip the section?
> > >
> > > It looks like mach_use_reliable_get_clock and
> > > src_use_reliable_get_clock could become a single field:
> > >
> > > * use_reliable_get_clock ("x-use-reliable-get-clock" property)
> > > set to true by default (set on DEFINE_PROP_BOOL parameter)
> > > * "x-use-reliable-get-clock" set to false by default on older
> > > machine-types
> > > * use_reliable_get_clock forced to false on kvmclock_realize() if
> > > !kvm_has_adjust_clock_stable()
> > > * kvmclock_reliable_get_clock.needed return s->use_reliable_get_clock
> >
> > No because use_reliable_get_clock can be false because the destination host
> > does not support kvm_has_adjust_clock_stable(), but the source host
> > supports it (so we want to use the value in the first incoming
> > migration).
> >
>
> Are you talking about the value of the fields on the source host,
> or on the destination host? In this case, you mean that
> use_reliable_get_clock should be false on the destination, right?
Yes, destination.
> > So the variable indicates whether the source host supported
> > migration, not whether the destination supports it.
>
> Good, that's the whole point of the field, isn't it? I will try
> to summarize all cases below. Please correct me if something is
> incorrect.
Yes. So in this case, the cleanup/simplification to use a single
variable is not possible (or maybe it is, but it just complicates
things).
> --------+------------+---------------+-----------+------------+
> | kvm_has_adj_clock_stable() | use_reliable_get_clock |
> 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 | (any) | (any) | false | false |
> --------+------------+---------------+-----------+------------+
>
> (*) I guess this is where things would break if we skip the
> subsection (and use a single field). If the subsection is
> skipped, use_reliable_get_clock would be set to true on the
> destination. (See my comment below on the new patch you sent)
Well, perhaps you can massage things so that it works
without sending subsection (that is):
* Send subsection only in case value is true.
* Do not send subsection in case value is false (then treat
non presence of subsection and mach_reliable_clock=true
to mean source did not support kvm_has_reliable_clock).
Once you agree on "send a subsection for both cases", then you
handle it.
The code is there, its simple, readable, so...
(what you can't do is to unify src_use_get_reliable_clock (remote
information), and local information). Because you need
to know both things to be able to handle the 4 cases properly on
incoming migration.
> (**) Destination can still do the right thing here, right?
>
>
>
> >
> [...]
> >
> > How about the following:
> >
> > 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 @@
> >
> > 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()) {
> > + 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;
> > +}
>
> I didn't review kvmclock_vm_state_change() and
> kvm_has_adjust_clock_stable(), but the rules to migrate
> src_use_reliable_get_clock look good now, and seem to match the
> table above.
Ok... Paolo, Radim?
Thanks Eduardo.
>
> > +
> > +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();
> > +}
> > +
> > 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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