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

[Dr. David Alan Gilbert]

* 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.

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