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

[Peter Xu <peterx@redhat.com>]

On Mon, Dec 04, 2023 at 04:31:56PM -0300, Fabiano Rosas wrote:
> 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.

What I meant is something like:

  runstate[100] = {"str1", 0x00, "str2", 0x00, ...}

Where str1 is runstate, and str2 can be either "0"/"1" to reflect suspended
value.  We define all the strings separated by 0x00, then IIUC we save the
most chars for potential future extension of this string.

Thanks,

-- 
Peter Xu