Re: [Qemu-devel] [PATCH] kvmclock: clarify usage of cpu_clean_all_dirty

[Paolo Bonzini]

Il 16/09/2014 18:07, Marcelo Tosatti ha scritto:
>> > The cpu_synchronize_all_states() call in kvmclock_vm_state_change() is
>> > needed to make env->tsc up to date with the value on the source, right?
> Its there to make sure the pair
> 
> env->tsc, s->clock = data.clock
> 
> are relative to point close in time.

Ok.  But why are they not close in time?

Could we have the opposite situation where env->tsc is loaded a long
time _after_ s->clock, and something breaks?

Also, there is no reason to do kvmclock_current_nsec() during normal
execution of the VM.  Is the s->clock sent by the source ever good
across migration, and could the source send kvmclock_current_nsec()
instead of whatever KVM_GET_CLOCK returns?

I don't understand this code very well, but it seems to me that the
migration handling and VM state change handler are mixed up...

Paolo

>> > But if the synchronize_all_states+clean_all_dirty pair runs on the
>> > source, why is the cpu_synchronize_all_states() call in
>> > qemu_savevm_state_complete() not enough?  It runs even later than
>> > kvmclock_vm_state_change.
> Because of the "pair of time values" explanation above.