On Thu, Nov 17, 2016 at 03:15:03PM +0100, Paolo Bonzini wrote:
>
>
> On 17/11/2016 15:11, 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.
> >
> >> + }
> >>
> >> - /* 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()?
Done. Added a new variable because the code was not complete
(stop/cont also runs locally).
> > What exactly ensures src_use_reliable_get_clock is correctly
> > initialized on the migration destination as well?
> >
> >> +
> >> + 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?
>
> This is what I asked for. :)
>
> However, I was proposing a different way to initialize
> src_use_reliable_get_clock. I still have to understand exactly how
> Marcelo's algorithm works because (based on the kvmclock code) it's more
> trick than it seems.
>
> Paolo
You asked me to return s->mach_use_reliable_get_clock:
>>> + * avoids reading kvmclock from memory.
>>> + */
>>> + if (s->mach_use_reliable_get_clock &&
>>> kvm_has_adjust_clock_stable()) {
>>> + s->src_use_reliable_get_clock = true;
>>> + }
>>> +
>>> + return s->src_use_reliable_get_clock;
>>> +}
>>
>> Here you can just return s->mach_use_reliable_get_clock.
>
> mach_use_reliable_get_clock can be true but host might not support it.
Yes, but the "needed" function is only required to avoid breaking
pc-i440fx-2.7 and earlier. If you return true here, you can still
migrate a "false" value for src_use_reliable_get_clock.
===================
I don't see why avoid the subsection, since new machine type is
incompatible anyway. So Eduardo on your suggestion to
skip sending the subsection, what is the advantage?
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