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

[<peng.hao2@zte.com.cn>]

> 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
>    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
I don't think I undo the previous patch. There are tow scenarios :

hotplugging memory happens after machine_init_done(). so the modified code is no 

influence.

hotplugged memory's VM is just migrated . on source its regions is less than 

supported by backend ,so on destination it should satisfy. During restoring VM's regions

may be more than supported by backend but after machine_init_done VM's regions



can be less than  supported by backend.
>          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)