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

[peng.hao2]

> > 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" <address@hidden> wrote:
> > > 
> > > > * Igor Mammedov (address@hidden) wrote:
> > > > > On Wed, 19 Jul 2017 23:17:32 +0800
> > > > > Peng Hao <address@hidden> 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.

some regions can not merge when just part of devices have realized.

when all devices are realized on destination, these regions  can be merged  
again 

 and the used slots can satisfy the vhost slot limit as on source.

in the restore stage the vm is not in running state, so don't compute if more 
than 

vhost slot limit when vm is not running. when a last device is realized,

all regions are merged to the slot number as on source. then the state of vm 

changes to running.

 it satisfies the vhost slot limit on source, so it should satify on 
destination.




> > > > > > 
> > > > > > Signed-off-by: Peng Hao <address@hidden>
> > > > > > Signed-off-by: Wang Yechao <address@hidden>
> > > > > > ---
> > > > > >  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 / address@hidden / Manchester, UK