On Mon, Dec 04, 2023 at 04:31:56PM -0300, Fabiano Rosas wrote:
> Peter Xu <pet...@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
