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Re: [Qemu-ppc] [Qemu-devel] [libvirt] [PATCH 1/2] numa: deprecate 'mem'

From: Daniel P . Berrangé
Subject: Re: [Qemu-ppc] [Qemu-devel] [libvirt] [PATCH 1/2] numa: deprecate 'mem' parameter of '-numa node' option
Date: Fri, 1 Mar 2019 17:48:06 +0000
User-agent: Mutt/1.11.3 (2019-02-01)

On Fri, Mar 01, 2019 at 06:33:28PM +0100, Igor Mammedov wrote:
> On Fri, 1 Mar 2019 15:49:47 +0000
> Daniel P. Berrangé <address@hidden> wrote:
> > On Fri, Mar 01, 2019 at 04:42:15PM +0100, Igor Mammedov wrote:
> > > The parameter allows to configure fake NUMA topology where guest
> > > VM simulates NUMA topology but not actually getting a performance
> > > benefits from it. The same or better results could be achieved
> > > using 'memdev' parameter. In light of that any VM that uses NUMA
> > > to get its benefits should use 'memdev' and to allow transition
> > > initial RAM to device based model, deprecate 'mem' parameter as
> > > its ad-hoc partitioning of initial RAM MemoryRegion can't be
> > > translated to memdev based backend transparently to users and in
> > > compatible manner (migration wise).
> > > 
> > > That will also allow to clean up a bit our numa code, leaving only
> > > 'memdev' impl. in place and several boards that use node_mem
> > > to generate FDT/ACPI description from it.  
> > 
> > Can you confirm that the  'mem' and 'memdev' parameters to -numa
> > are 100% live migration compatible in both directions ?  Libvirt
> > would need this to be the case in order to use the 'memdev' syntax
> > instead.
> Unfortunately they are not migration compatible in any direction,
> if it where possible to translate them to each other I'd alias 'mem'
> to 'memdev' without deprecation. The former sends over only one
> MemoryRegion to target, while the later sends over several (one per
> memdev).

If we can't migration from one to the other, then we can not deprecate
the existing 'mem' syntax. Even if libvirt were to provide a config
option to let apps opt-in to the new syntax, we need to be able to
support live migration of existing running VMs indefinitely. Effectively
this means we need the to keep 'mem' support forever, or at least such
a long time that it effectively means forever.

So I think this patch has to be dropped & replaced with one that
simply documents that memdev syntax is preferred.

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