Re: [Qemu-devel] [PATCH v2 2/5] Revert "migration: move only_migratable to MigrationState"

[Daniel P . Berrangé]

On Mon, Apr 01, 2019 at 01:31:47PM +0200, Markus Armbruster wrote:
> Daniel P. Berrangé <address@hidden> writes:
> 
> > On Mon, Apr 01, 2019 at 10:27:49AM +0100, Dr. David Alan Gilbert wrote:
> >> * Markus Armbruster (address@hidden) wrote:
> >> > This reverts commit 3df663e575f1876d7f3bc684f80e72fca0703d39.
> >> > This reverts commit b605c47b57b58e61a901a50a0762dccf43d94783.
> >> > 
> >> > Command line option --only-migratable is for disallowing any
> >> > configuration that can block migration.
> >> > 
> >> > Initially, --only-migratable set global variable @only_migratable.
> >> > 
> >> > Commit 3df663e575 "migration: move only_migratable to MigrationState"
> >> > replaced it by MigrationState member @only_migratable.  That was a
> >> > mistake.
> >> > 
> >> > First, it doesn't make sense on the design level.  MigrationState
> >> > captures the state of an individual migration, but --only-migratable
> >> > isn't a property of an individual migration, it's a restriction on
> >> > QEMU configuration.  With fault tolerance, we could have several
> >> > migrations at once.  --only-migratable would certainly protect all of
> >> > them.  Storing it in MigrationState feels inappropriate.
> >> > 
> >> > Second, it contributes to a dependency cycle that manifests itself as
> >> > a bug now.
> >> > 
> >> > Putting @only_migratable into MigrationState means its available only
> >> > after migration_object_init().
> >> > 
> >> > We can't set it before migration_object_init(), so we delay setting it
> >> > with a global property (this is fixup commit b605c47b57 "migration:
> >> > fix handling for --only-migratable").
> >> > 
> >> > We can't get it before migration_object_init(), so anything that uses
> >> > it can only run afterwards.
> >> > 
> >> > Since migrate_add_blocker() needs to obey --only-migratable, any code
> >> > adding migration blockers can run only afterwards.  This contributes
> >> > to the following dependency cycle:
> >> > 
> >> > * configure_blockdev() must run before machine_set_property()
> >> >   so machine properties can refer to block backends
> >> > 
> >> > * machine_set_property() before configure_accelerator()
> >> >   so machine properties like kvm-irqchip get applied
> >> > 
> >> > * configure_accelerator() before migration_object_init()
> >> >   so that Xen's accelerator compat properties get applied.
> >> > 
> >> > * migration_object_init() before configure_blockdev()
> >> >   so configure_blockdev() can add migration blockers
> >> > 
> >> > The cycle was closed when recent commit cda4aa9a5a0 "Create block
> >> > backends before setting machine properties" added the first
> >> > dependency, and satisfied it by violating the last one.  Broke block
> >> > backends that add migration blockers.
> >> > 
> >> > Moving @only_migratable into MigrationState was a mistake.  Revert it.
> >> > 
> >> > This doesn't quite break the "migration_object_init() before
> >> > configure_blockdev() dependency, since migrate_add_blocker() still has
> >> > another dependency on migration_object_init().  To be addressed the
> >> > next commit
> >> > 
> >> > Conflicts:
> >> >  include/migration/misc.h
> >> >  migration/migration.c
> >> >  migration/migration.h
> >> >  vl.c
> >> > 
> >> > Signed-off-by: Markus Armbruster <address@hidden>
> >> > ---
> >> >  include/sysemu/sysemu.h | 1 +
> >> >  migration/migration.c   | 5 ++---
> >> >  migration/migration.h   | 3 ---
> >> >  migration/savevm.c      | 2 +-
> >> >  vl.c                    | 9 ++-------
> >> >  5 files changed, 6 insertions(+), 14 deletions(-)
> >> > 
> >> > diff --git a/include/sysemu/sysemu.h b/include/sysemu/sysemu.h
> >> > index 6065d9e420..5f133cae83 100644
> >> > --- a/include/sysemu/sysemu.h
> >> > +++ b/include/sysemu/sysemu.h
> >> > @@ -14,6 +14,7 @@
> >> >  /* vl.c */
> >> >  
> >> >  extern const char *bios_name;
> >> > +extern int only_migratable;
> >> >  extern const char *qemu_name;
> >> >  extern QemuUUID qemu_uuid;
> >> >  extern bool qemu_uuid_set;
> >> > diff --git a/migration/migration.c b/migration/migration.c
> >> > index 69f75124c9..f6076e5295 100644
> >> > --- a/migration/migration.c
> >> > +++ b/migration/migration.c
> >> > @@ -1707,7 +1707,7 @@ static GSList *migration_blockers;
> >> >  
> >> >  int migrate_add_blocker(Error *reason, Error **errp)
> >> >  {
> >> > -    if (migrate_get_current()->only_migratable) {
> >> > +    if (only_migratable) {
> >> >          error_propagate_prepend(errp, error_copy(reason),
> >> >                                  "disallowing migration blocker "
> >> >                                  "(--only_migratable) for: ");
> >> > @@ -3337,7 +3337,7 @@ void migration_global_dump(Monitor *mon)
> >> >      monitor_printf(mon, "store-global-state: %s\n",
> >> >                     ms->store_global_state ? "on" : "off");
> >> >      monitor_printf(mon, "only-migratable: %s\n",
> >> > -                   ms->only_migratable ? "on" : "off");
> >> > +                   only_migratable ? "on" : "off");
> >> >      monitor_printf(mon, "send-configuration: %s\n",
> >> >                     ms->send_configuration ? "on" : "off");
> >> >      monitor_printf(mon, "send-section-footer: %s\n",
> >> > @@ -3352,7 +3352,6 @@ void migration_global_dump(Monitor *mon)
> >> >  static Property migration_properties[] = {
> >> >      DEFINE_PROP_BOOL("store-global-state", MigrationState,
> >> >                       store_global_state, true),
> >> > -    DEFINE_PROP_BOOL("only-migratable", MigrationState, 
> >> > only_migratable, false),
> >> 
> >> So I'm generally OK at this patch, but I'm worried whether this
> >> is API?
> 
> Uh, forgot to discuss this in my commit message, sorry!
> 
> > IIUC, old code can use either
> >
> >   -global migration.only-migratable=true
> >   -only-migratable
> >
> > After this patch only
> >
> >   -only-migratable
> >
> > is valid. So from that POV it is an API break. 
> >
> > From libvirt's POV this is a non-issue as we never use either of these 
> > options.
> > We did have a BZ filed to support it, but never did.
> >
> > To avoid the API break I guess there would been to be a special case early
> > loop iterating over argv[] looking for '-global 
> > migration.only-migratable=true',
> > making that set the static 'bool only_migratable', instead of letting it get
> > handled by the object infra.
> 
> Is this is worth the trouble?

Probably not.

> -global migration.only-migratable=on is entirely undocumented.  The
> property appears in -device migration,help and device-list-properties,
> but that's all.
> 
> Why would any sane person risk using arcane & undocumented -global
> migration.only-migratable=on instead of the obvious & documented
> -only-migratable?

Given the combination of no known users, current broken impl, and its
existance not documented, I think there's a reasonable case for just
removing it.

Regards,
Daniel
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