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

[peng.hao2]

* Michael S. Tsirkin (address@hidden) wrote:





>> 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

>OK, but that means that a region can incorporate multiple
>RAMBlocks though? Hmm that's not fun on postcopy.

>> so it does not affect migration normally.

>Well, why? What's required - if the region sizes/lengths/orders
>are different on the source and destination does it matter - if
>it does then that means we have a problem, since that heuristic
>is non-deterministic.
Yes,I encounter the different  region creation orders  on the source and 
destination

if you add dimms on the source before migration. so I just distinguish 
according to 

vm state.

>Dave


> 
> > 
> > > 
> > > > > 
> > > > > 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
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK