[Qemu-devel] Re: [PATCH v4] savevm: Fix no_migrate

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jan 10, 2011 at 10:47:58AM -0700, Alex Williamson wrote:
> On Sun, 2011-01-09 at 12:47 +0200, Michael S. Tsirkin wrote:
> > On Fri, Jan 07, 2011 at 03:13:25PM -0700, Alex Williamson wrote:
> > > The no_migrate save state flag is currently only checked in the
> > > last phase of migration.  This means that we potentially waste
> > > a lot of time and bandwidth with the live state handlers before
> > > we ever check the no_migrate flags.  The error message printed
> > > when we catch a non-migratable device doesn't get printed for
> > > a detached migration.  And, no_migrate does nothing to prevent
> > > an incoming migration to a target that includes a non-migratable
> > > device.  This attempts to fix all of these.
> > > 
> > > One notable difference in behavior is that an outgoing migration
> > > now checks for non-migratable devices before ever connecting to
> > > the target system.  This means the target will remain listening
> > > rather than exit from failure.
> > > 
> > > Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> > 
> > Minor nit:
> > 
> > The API of qemu_savevm_state_blocked is a bit strange.
> > functions should either return 0/1 values or 0 on success
> > negative on failure or a bool.
> > This API asks "is it blocked" and returns -EINVAL to
> > mean "yes".  _blocked is also a bit ambigious:
> > it seems to imply a temporary condition.
> 
> But it very well could be a temporary condition.  If I have an assigned
> device, it will block migration/savevm, but removing the device allows
> it to proceed.

Well to me blocked it means migration will not fail, but will be blocked
and unblocked later when device is removed.

> > How about we reverse the logic, call the new API
> > qemu_savevm_state_supported, qemu_savevm_state_enabled,
> > something like this?
> > Then you can return 0 if migration is possible,
> > -1 if not.
> 
> I tend to like qemu_savevm_state_blocked() as "supported" and "enabled"
> invoke different ideas for me.  I will concede that the errno return
> value is a little awkward.  How about if I make it a bool function
> instead?  Thanks,
> 
> Alex

That's better.

> > > ---
> > > 
> > > v4:
> > >   - fix braces noted by Jan
> > >   - return error from qemu_savevm_state_blocked rather than fixed EINVAL
> > >     at qemu_loadvm_state(), since it'a already using errno values
> > > 
> > > v3:
> > > 
> > > Daniel, adding you to see if libvirt cares about the difference in
> > > whether the target exits on migration failure as noted above.
> > > 
> > > Also adding Michael as he's complained about the no_migrate check
> > > happening so late in the process.
> > > 
> > >  migration.c |    4 ++++
> > >  savevm.c    |   41 +++++++++++++++++++++++++++--------------
> > >  sysemu.h    |    1 +
> > >  3 files changed, 32 insertions(+), 14 deletions(-)
> > > 
> > > diff --git a/migration.c b/migration.c
> > > index e5ba51c..d593b1d 100644
> > > --- a/migration.c
> > > +++ b/migration.c
> > > @@ -88,6 +88,10 @@ int do_migrate(Monitor *mon, const QDict *qdict, QObject **ret_data)
> > >          return -1;
> > >      }
> > >  
> > > +    if (qemu_savevm_state_blocked(mon)) {
> > > +        return -1;
> > > +    }
> > > +
> > >      if (strstart(uri, "tcp:", &p)) {
> > >          s = tcp_start_outgoing_migration(mon, p, max_throttle, detach,
> > >                                           blk, inc);
> > > diff --git a/savevm.c b/savevm.c
> > > index 90aa237..34c0d1a 100644
> > > --- a/savevm.c
> > > +++ b/savevm.c
> > > @@ -1401,19 +1401,13 @@ static int vmstate_load(QEMUFile *f, SaveStateEntry *se, int version_id)
> > >      return vmstate_load_state(f, se->vmsd, se->opaque, version_id);
> > >  }
> > >  
> > > -static int vmstate_save(QEMUFile *f, SaveStateEntry *se)
> > > +static void vmstate_save(QEMUFile *f, SaveStateEntry *se)
> > >  {
> > > -    if (se->no_migrate) {
> > > -        return -1;
> > > -    }
> > > -
> > >      if (!se->vmsd) {         /* Old style */
> > >          se->save_state(f, se->opaque);
> > > -        return 0;
> > > +        return;
> > >      }
> > >      vmstate_save_state(f,se->vmsd, se->opaque);
> > > -
> > > -    return 0;
> > >  }
> > >  
> > >  #define QEMU_VM_FILE_MAGIC           0x5145564d
> > > @@ -1427,6 +1421,20 @@ static int vmstate_save(QEMUFile *f, SaveStateEntry *se)
> > >  #define QEMU_VM_SECTION_FULL         0x04
> > >  #define QEMU_VM_SUBSECTION           0x05
> > >  
> > > +int qemu_savevm_state_blocked(Monitor *mon)
> > > +{
> > > +    SaveStateEntry *se;
> > > +
> > > +    QTAILQ_FOREACH(se, &savevm_handlers, entry) {
> > > +        if (se->no_migrate) {
> > > +            monitor_printf(mon, "state blocked by non-migratable device '%s'\n",
> > > +                           se->idstr);
> > > +            return -EINVAL;
> > > +        }
> > > +    }
> > > +    return 0;
> > > +}
> > > +
> > >  int qemu_savevm_state_begin(Monitor *mon, QEMUFile *f, int blk_enable,
> > >                              int shared)
> > >  {
> > > @@ -1508,7 +1516,6 @@ int qemu_savevm_state_iterate(Monitor *mon, QEMUFile *f)
> > >  int qemu_savevm_state_complete(Monitor *mon, QEMUFile *f)
> > >  {
> > >      SaveStateEntry *se;
> > > -    int r;
> > >  
> > >      cpu_synchronize_all_states();
> > >  
> > > @@ -1541,11 +1548,7 @@ int qemu_savevm_state_complete(Monitor *mon, QEMUFile *f)
> > >          qemu_put_be32(f, se->instance_id);
> > >          qemu_put_be32(f, se->version_id);
> > >  
> > > -        r = vmstate_save(f, se);
> > > -        if (r < 0) {
> > > -            monitor_printf(mon, "cannot migrate with device '%s'\n", se->idstr);
> > > -            return r;
> > > -        }
> > > +        vmstate_save(f, se);
> > >      }
> > >  
> > >      qemu_put_byte(f, QEMU_VM_EOF);
> > > @@ -1575,6 +1578,11 @@ static int qemu_savevm_state(Monitor *mon, QEMUFile *f)
> > >      saved_vm_running = vm_running;
> > >      vm_stop(0);
> > >  
> > > +    ret = qemu_savevm_state_blocked(mon);
> > > +    if (ret < 0) {
> > > +        goto out;
> > > +    }
> > > +
> > >      ret = qemu_savevm_state_begin(mon, f, 0, 0);
> > >      if (ret < 0)
> > >          goto out;
> > > @@ -1692,6 +1700,11 @@ int qemu_loadvm_state(QEMUFile *f)
> > >      unsigned int v;
> > >      int ret;
> > >  
> > > +    ret = qemu_savevm_state_blocked(default_mon);
> > > +    if (ret < 0) {
> > > +        return ret;
> > > +    }
> > > +
> > >      v = qemu_get_be32(f);
> > >      if (v != QEMU_VM_FILE_MAGIC)
> > >          return -EINVAL;
> > > diff --git a/sysemu.h b/sysemu.h
> > > index e728ea1..eefaba5 100644
> > > --- a/sysemu.h
> > > +++ b/sysemu.h
> > > @@ -75,6 +75,7 @@ void qemu_announce_self(void);
> > >  
> > >  void main_loop_wait(int nonblocking);
> > >  
> > > +int qemu_savevm_state_blocked(Monitor *mon);
> > >  int qemu_savevm_state_begin(Monitor *mon, QEMUFile *f, int blk_enable,
> > >                              int shared);
> > >  int qemu_savevm_state_iterate(Monitor *mon, QEMUFile *f);
> 
>