Re: [Qemu-devel] [PATCH V2] vhost: fix a migration failed because of vhost region merge

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

On Mon, Jul 24, 2017 at 11:14:19AM +0200, Igor Mammedov wrote:
> On Sun, 23 Jul 2017 20:46:11 +0800
> Peng Hao <peng.hao2@zte.com.cn> wrote:
> 
> > When a guest that has several hotplugged dimms is migrated, on
> > destination it will fail to resume. Because regions on source
> > are merged and on destination the order of realizing devices
> > is different from on source with dimms, so when part of devices
> > are realizd some region can not be merged.That may be more than
> > vhost slot limit.
> > 
> > Signed-off-by: Peng Hao <peng.hao2@zte.com.cn>
> > Signed-off-by: Wang Yechao <wang.yechao255@zte.com.cn>
> > ---
> >  hw/mem/pc-dimm.c        | 2 +-
> >  include/sysemu/sysemu.h | 1 +
> >  vl.c                    | 5 +++++
> >  3 files changed, 7 insertions(+), 1 deletion(-)
> > 
> > diff --git a/hw/mem/pc-dimm.c b/hw/mem/pc-dimm.c
> > index ea67b46..13f3db5 100644
> > --- a/hw/mem/pc-dimm.c
> > +++ b/hw/mem/pc-dimm.c
> > @@ -101,7 +101,7 @@ void pc_dimm_memory_plug(DeviceState *dev, MemoryHotplugState *hpms,
> >          goto out;
> >      }
> >  
> > -    if (!vhost_has_free_slot()) {
> > +    if (!vhost_has_free_slot() && qemu_is_machine_init_done()) {
> >          error_setg(&local_err, "a used vhost backend has no free"
> >                                 " memory slots left");
> that doesn't fix issue,
>    1st: number of used entries is changing after machine_init_done() is called
>         as regions continue to mapped/unmapped during runtime

But that's fine, we want hotplug to fail if we can not guarantee vhost
will work.

>    2nd: it brings regression and allows to start QEMU with number memory
>         regions more than supported by backend, which combined with missing
>         error handling in vhost will lead to qemu crashes or obscure bugs in
>         guest breaking vhost enabled drivers.
>         i.e. patch undoes what were fixed by
>         https://lists.gnu.org/archive/html/qemu-devel/2015-10/msg00789.html

Why does it? The issue you fixed there is hotplug, and that means
pc_dimm_memory_plug called after machine done.

> 
> >          goto out;
> > diff --git a/include/sysemu/sysemu.h b/include/sysemu/sysemu.h
> > index b213696..48228ad 100644
> > --- a/include/sysemu/sysemu.h
> > +++ b/include/sysemu/sysemu.h
> > @@ -88,6 +88,7 @@ void qemu_system_guest_panicked(GuestPanicInformation *info);
> >  void qemu_add_exit_notifier(Notifier *notify);
> >  void qemu_remove_exit_notifier(Notifier *notify);
> >  
> > +bool qemu_is_machine_init_done(void);
> >  void qemu_add_machine_init_done_notifier(Notifier *notify);
> >  void qemu_remove_machine_init_done_notifier(Notifier *notify);
> >  
> > diff --git a/vl.c b/vl.c
> > index fb6b2ef..43aee22 100644
> > --- a/vl.c
> > +++ b/vl.c
> > @@ -2681,6 +2681,11 @@ static void qemu_run_exit_notifiers(void)
> >  
> >  static bool machine_init_done;
> >  
> > +bool qemu_is_machine_init_done(void)
> > +{
> > +    return machine_init_done;
> > +}
> > +
> >  void qemu_add_machine_init_done_notifier(Notifier *notify)
> >  {
> >      notifier_list_add(&machine_init_done_notifiers, notify);