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

Pankaj Gupta
2016-12-12T07:37:02Z

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);
>
>
>
>