Re: [Qemu-devel] [PATCH] migration: Do not re-read the clock on pre_save in case of paused guest

[Marcelo Tosatti <mtosatti@redhat.com>]

Looks good.

Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>

On Thu, Aug 29, 2019 at 06:18:42PM -0300, Eduardo Habkost wrote:
> CCing Marcelo, who wrote kvm_update_clock() and
> kvmclock_pre_save().
> 
> On Thu, Aug 29, 2019 at 06:07:11PM -0300, Maxiwell S. Garcia wrote:
> > The clock move makes the guest knows about the paused time between the
> > 'stop' and 'migrate' commands. This is an issue in an already-paused
> > VM because some side effects, like process stalls, could happen
> > after migration.
> > 
> > So, this patch checks the runstate of guest in the pre_save handler and
> > do not re-reads the clock in case of paused state (cold migration).
> > 
> > Signed-off-by: Maxiwell S. Garcia <maxiwell@linux.ibm.com>
> > ---
> >  hw/i386/kvm/clock.c | 15 +++++++++++----
> >  1 file changed, 11 insertions(+), 4 deletions(-)
> > 
> > diff --git a/hw/i386/kvm/clock.c b/hw/i386/kvm/clock.c
> > index 80c133a724..2c59b6894b 100644
> > --- a/hw/i386/kvm/clock.c
> > +++ b/hw/i386/kvm/clock.c
> > @@ -41,6 +41,9 @@ typedef struct KVMClockState {
> >      uint64_t clock;
> >      bool clock_valid;
> >  
> > +    /* whether the 'clock' value was obtained in the 'paused' state */
> > +    bool runstate_paused;
> > +
> >      /* whether machine type supports reliable KVM_GET_CLOCK */
> >      bool mach_use_reliable_get_clock;
> >  
> > @@ -202,6 +205,8 @@ static void kvmclock_vm_state_change(void *opaque, int running,
> >              return;
> >          }
> >  
> > +        s->runstate_paused = runstate_check(RUN_STATE_PAUSED);
> > +
> >          kvm_synchronize_all_tsc();
> >  
> >          kvm_update_clock(s);
> > @@ -260,9 +265,9 @@ static int kvmclock_pre_load(void *opaque)
> >  }
> >  
> >  /*
> > - * When migrating, read the clock just before migration,
> > - * so that the guest clock counts during the events
> > - * between:
> > + * When migrating a running guest, read the clock just
> > + * before migration, so that the guest clock counts
> > + * during the events between:
> >   *
> >   *  * vm_stop()
> >   *  *
> > @@ -277,7 +282,9 @@ static int kvmclock_pre_save(void *opaque)
> >  {
> >      KVMClockState *s = opaque;
> >  
> > -    kvm_update_clock(s);
> > +    if (!s->runstate_paused) {
> > +        kvm_update_clock(s);
> > +    }
> >  
> >      return 0;
> >  }
> > -- 
> > 2.20.1
> > 
> 
> -- 
> Eduardo