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

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

On Thu, Jul 20, 2017 at 10:57:57AM +0800, peng.hao2@zte.com.cn wrote:
> 原始邮件
> 发件人： <mst@redhat.com>;
> 收件人： <imammedo@redhat.com>;
> 抄送人： <dgilbert@redhat.com>; <marcandre.lureau@redhat.com>;
> <maxime.coquelin@redhat.com>;彭浩10096742;王业超10154425;
> <qemu-devel@nongnu.org>;
> 日期：2017年07月19日 23:53
> 主题：Re: [Qemu-devel] [PATCH] vhost: fix a migration failed because ofvhost
> region merge
> 
> 
> On Wed, Jul 19, 2017 at 03:24:27PM +0200, Igor Mammedov wrote:
> > On Wed, 19 Jul 2017 12:46:13 +0100
> > "Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:
> > 
> > > * Igor Mammedov (imammedo@redhat.com) wrote:
> > > > On Wed, 19 Jul 2017 23:17:32 +0800
> > > > Peng Hao <peng.hao2@zte.com.cn> wrote:
> > > >   
> > > > > When a guest that has several hotplugged dimms is migrated, in
> > > > > destination host it will fail to resume. Because vhost regions of
> > > > > several dimms in source host are merged and in the restore stage
> > > > > in destination host it computes whether more than vhost slot limit
> > > > > before merging vhost regions of several dimms.  
> > > > could you provide a bit more detailed description of the problem
> > > > including command line+used device_add commands on source and
> > > > command line on destination?  
> > > 
> > > (ccing in Marc Andre and Maxime)
> > > 
> > > Hmm, I'd like to understade the situation where you get merging between
> > > RAMBlocks; that complicates some stuff for postcopy.
> > and probably inconsistent merging breaks vhost as well
> > 
> > merging might happen if regions are adjacent or overlap
> > but for that to happen merged regions must have equal
> > distance between their GPA:HVA pairs, so that following
> > translation would work:
> > 
> > if gva in regionX[gva_start, len, hva_start]
> >    hva = hva_start + gva - gva_start
> > 
> > while GVA of regions is under QEMU control and deterministic
> > HVA is not, so in migration case merging might happen on source
> > side but not on destination, resulting in different memory maps.
> > 
> > Maybe Michael might know details why migration works in vhost usecase,
> > but I don't see vhost sending any vmstate data.
> 
> We aren't merging ramblocks at all.
> When we are passing blocks A and B to vhost, if we see that
> 
> hvaB=hvaA + lenA
> gpaB=gpaA + lenA
> 
> then we can improve performance a bit by passing a single
> chunk to vhost: hvaA,gpaA,lena+lenB
> 
> 
> so it does not affect migration normally.
> 
> ----- I think it is like this:
> 
> in source;                                       in destination:(restore)
> 
> realize device 1                              realize device 1
> 
> realize device 2                              realize dimm 0
> 
>      ...                                               realize dimm1
> 
>                                                        ....
> 
> realize device n                              realize dimmx
> 
>                                                        realize  device m
> 
> realize dimm0                                  .....
> 
> realize dimm1                                  .....
> 
> ......                                                  .....
> 
> realize dimmx                                realize device n
> 
> 
> In restore stage ,the sort of realizing device  is different from starting vm
> because of adding dimms.
> 
> So it may in some stage during restoring can't merge vhost regions.

If you run over the number of regions supported by vhost on destination
then you won't be able to start a VM there until you disable vhost.



> 
> 
> 
> 
> 
> > 
> > > 
> > > > > 
> > > > > 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 +-
> > > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > > 
> > > > > diff --git a/hw/mem/pc-dimm.c b/hw/mem/pc-dimm.c
> > > > > index ea67b46..bb0fa08 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() && runstate_is_running()) {
> > > > >          error_setg(&local_err, "a used vhost backend has no free"
> > > > >                                 " memory slots left");
> > > > >          goto out;  
> > > 
> > > Even this produces the wrong error message in this case,
> > > it also makes me think if the existing code should undo a lot of
> > > the object_property_set's that happen.
> > > 
> > > Dave
> > > > 
> > > >   
> > > --
> > > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> 
> 
>