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Re: [Qemu-ppc] [Qemu-devel] [PATCH v3 3/3] pc-dimm: factor out address s

From: Pankaj Gupta
Subject: Re: [Qemu-ppc] [Qemu-devel] [PATCH v3 3/3] pc-dimm: factor out address space logic into MemoryDevice code
Date: Wed, 25 Apr 2018 09:56:49 -0400 (EDT)

> > > > >     
> > > > >> +
> > > > >> +    memory_region_add_subregion(&hpms->mr, addr - hpms->base, mr);
> > > > > missing vmstate registration?
> > > > 
> > > > Missed this one: To be called by the caller. Important because e.g. for
> > > > virtio-pmem we don't want this (I assume :) ).
> > > if pmem isn't on shared storage, then We'd probably want to migrate
> > > it as well, otherwise target would experience data loss.
> > > Anyways, I'd just reat it as normal RAM in migration case
> > 
> > Main difference between RAM and pmem it acts like combination of RAM and
> > disk.
> > Saying this, in normal use-case size would be 100 GB's - few TB's range.
> > I am not sure we really want to migrate it for non-shared storage use-case.
> with non shared storage you'd have to migrate it target host but
> with shared storage it might be possible to flush it and use directly
> from target host. That probably won't work right out of box and would
> need some sort of synchronization between src/dst hosts.

Shared storage should work out of the box. Only thing is data in destination
host will be cache cold and existing pages in cache should be invalidated 
But if we migrate entire fake DAX RAMstate it will populate destination host 
cache including pages while were idle in source host. This would unnecessarily 
create entropy in destination host. 

To me this feature don't make much sense. Problem which we are solving is:
Efficiently use guest RAM. 

> The same applies to nv/pc-dimm as well, as backend file easily could be
> on pmem storage as well.

Are you saying backing file is in actual actual nvdimm hardware? we don't need 
emulation at all. 

> Maybe for now we should migrate everything so it would work in case of
> non shared NVDIMM on host. And then later add migration-less capability
> to all of them.

not sure I agree.

> > One reason why nvdimm added vmstate info could be: still there would be
> > transient
> > writes in memory with fake DAX and there is no way(till now) to flush the
> > guest
> > writes. But with virtio-pmem we can flush such writes before migration and
> > automatically
> > at destination host with shared disk we will have updated data.
> nvdimm has concept of flush address hint (may be not implemented in qemu yet)
> but it can flush. The only reason I'm buying into virtio-mem idea
> is that would allow async flush queues which would reduce number
> of vmexits.

Thats correct.



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