Re: [PATCH V6 05/14] migration: propagate suspended runstate

[Fabiano Rosas]

Peter Xu <peterx@redhat.com> writes:

> On Fri, Dec 01, 2023 at 11:23:33AM -0500, Steven Sistare wrote:
>> >> @@ -109,6 +117,7 @@ static int global_state_post_load(void *opaque, int 
>> >> version_id)
>> >>          return -EINVAL;
>> >>      }
>> >>      s->state = r;
>> >> +    vm_set_suspended(s->vm_was_suspended || r == RUN_STATE_SUSPENDED);
>> > 
>> > IIUC current vm_was_suspended (based on my read of your patch) was not the
>> > same as a boolean representing "whether VM is suspended", but only a
>> > temporary field to remember that for a VM stop request.  To be explicit, I
>> > didn't see this flag set in qemu_system_suspend() in your previous patch.
>> > 
>> > If so, we can already do:
>> > 
>> >   vm_set_suspended(s->vm_was_suspended);
>> > 
>> > Irrelevant of RUN_STATE_SUSPENDED?
>> 
>> We need both terms of the expression.
>> 
>> If the vm *is* suspended (RUN_STATE_SUSPENDED), then vm_was_suspended = 
>> false.
>> We call global_state_store prior to vm_stop_force_state, so the incoming
>> side sees s->state = RUN_STATE_SUSPENDED and s->vm_was_suspended = false.
>
> Right.
>
>> However, the runstate is RUN_STATE_INMIGRATE.  When incoming finishes by
>> calling vm_start, we need to restore the suspended state.  Thus in 
>> global_state_post_load, we must set vm_was_suspended = true.
>
> With above, shouldn't global_state_get_runstate() (on dest) fetch SUSPENDED
> already?  Then I think it should call vm_start(SUSPENDED) if to start.
>
> Maybe you're talking about the special case where autostart==false?  We
> used to have this (existing process_incoming_migration_bh()):
>
>     if (!global_state_received() ||
>         global_state_get_runstate() == RUN_STATE_RUNNING) {
>         if (autostart) {
>             vm_start();
>         } else {
>             runstate_set(RUN_STATE_PAUSED);
>         }
>     }
>
> If so maybe I get you, because in the "else" path we do seem to lose the
> SUSPENDED state again, but in that case IMHO we should logically set
> vm_was_suspended only when we "lose" it - we didn't lose it during
> migration, but only until we decided to switch to PAUSED (due to
> autostart==false). IOW, change above to something like:
>
>     state = global_state_get_runstate();
>     if (!global_state_received() || runstate_is_alive(state)) {
>         if (autostart) {
>             vm_start(state);
>         } else {
>             if (runstate_is_suspended(state)) {
>                 /* Remember suspended state before setting system to STOPed */
>                 vm_was_suspended = true;
>             }
>             runstate_set(RUN_STATE_PAUSED);
>         }
>     }
>
> It may or may not have a functional difference even if current patch,
> though.  However maybe clearer to follow vm_was_suspended's strict
> definition.
>
>> 
>> If the vm *was* suspended, but is currently stopped (eg RUN_STATE_PAUSED),
>> then vm_was_suspended = true.  Migration from that state sets
>> vm_was_suspended = s->vm_was_suspended = true in global_state_post_load and 
>> ends with runstate_set(RUN_STATE_PAUSED).
>> 
>> I will add a comment here in the code.
>>  
>> >>      return 0;
>> >>  }
>> >> @@ -134,6 +143,7 @@ static const VMStateDescription vmstate_globalstate = 
>> >> {
>> >>      .fields = (VMStateField[]) {
>> >>          VMSTATE_UINT32(size, GlobalState),
>> >>          VMSTATE_BUFFER(runstate, GlobalState),
>> >> +        VMSTATE_BOOL(vm_was_suspended, GlobalState),
>> >>          VMSTATE_END_OF_LIST()
>> >>      },
>> >>  };
>> > 
>> > I think this will break migration between old/new, unfortunately.  And
>> > since the global state exist mostly for every VM, all VM setup should be
>> > affected, and over all archs.
>> 
>> Thanks, I keep forgetting that my binary tricks are no good here.  However,
>> I have one other trick up my sleeve, which is to store vm_was_running in
>> global_state.runstate[strlen(runstate) + 2].  It is forwards and backwards
>> compatible, since that byte is always 0 in older qemu.  It can be implemented
>> with a few lines of code change confined to global_state.c, versus many 
>> lines 
>> spread across files to do it the conventional way using a compat property and
>> a subsection.  Sound OK?  
>
> Tricky!  But sounds okay to me.  I think you're inventing some of your own
> way of being compatible, not relying on machine type as a benefit.  If go
> this route please document clearly on the layout and also what it looked
> like in old binaries.
>
> I think maybe it'll be good to keep using strings, so in the new binaries
> we allow >1 strings, then we define properly on those strings (index 0:
> runstate, existed since start; index 2: suspended, perhaps using "1"/"0" to
> express, while 0x00 means old binary, etc.).
>
> I hope this trick will need less code than the subsection solution,
> otherwise I'd still consider going with that, which is the "common
> solution".
>
> Let's also see whether Juan/Fabiano/others has any opinions.

Can't we pack the structure and just go ahead and slash 'runstate' in
half? That would claim some unused bytes for future backward
compatibility issues.