Re: [Qemu-arm] [Qemu-devel] [libvirt] [PATCH 1/2] numa: deprecate 'mem' parameter of '-numa node' option

[Michal Privoznik]

On 3/4/19 3:16 PM, Igor Mammedov wrote:
> On Mon, 4 Mar 2019 12:39:08 +0000
> Daniel P. Berrangé <address@hidden> wrote:
>
> > On Mon, Mar 04, 2019 at 01:25:07PM +0100, Igor Mammedov wrote:
> > > On Mon, 04 Mar 2019 08:13:53 +0100
> > > Markus Armbruster <address@hidden> wrote:
> > >    
> > > > Daniel P. Berrangé <address@hidden> writes:
> > > >    
> > > > > 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.
> > > >
> > > > We have this habit of postulating absolutes like "can not deprecate"
> > > > instead of engaging with the tradeoffs.  We need to kick it.
> > > >
> > > > So let's have an actual look at the tradeoffs.
> > > >
> > > > We don't actually "support live migration of existing running VMs
> > > > indefinitely".
> > > >
> > > > We support live migration to any newer version of QEMU that still
> > > > supports the machine type.
> > > >
> > > > We support live migration to any older version of QEMU that already
> > > > supports the machine type and all the devices the machine uses.
> > > >
> > > > Aside: "support" is really an honest best effort here.  If you rely on
> > > > it, use a downstream that puts in the (substantial!) QA work real
> > > > support takes.
> > > >
> > > > Feature deprecation is not a contract to drop the feature after two
> > > > releases, or even five.  It's a formal notice that users of the feature
> > > > should transition to its replacement in an orderly manner.
> > > >
> > > > If I understand Igor correctly, all users should transition away from
> > > > outdated NUMA configurations at least for new VMs in an orderly manner.
> > > Yes, we can postpone removing options until there are machines type
> > > versions that were capable to use it (unfortunate but probably
> > > unavoidable unless there is a migration trick to make transition
> > > transparent) but that should not stop us from disabling broken
> > > options on new machine types at least.
> > >
> > > This series can serve as formal notice with follow up disabling of
> > > deprecated options for new machine types. (As Thomas noted, just warnings
> > > do not work and users continue to use broken features regardless whether
> > > they are don't know about issues or aware of it [*])
> > >
> > > Hence suggested deprecation approach and enforced rejection of legacy
> > > numa options for new machine types in 2 releases so users would stop
> > > using them eventually.
> >
> > When we deprecate something, we need to have a way for apps to use the
> > new alternative approach *at the same time*.  So even if we only want to
> > deprecate for new machine types, we still have to first solve the problem
> > of how mgmt apps will introspect QEMU to learn which machine types expect
> > the new options.
> I'm not aware any mechanism to introspect machine type options (existing
> or something being developed). Are/were there any ideas about it that were
> discussed in the past?
>
> Aside from developing a new mechanism what are alternative approaches?
> I mean when we delete deprecated CLI option, how it's solved on libvirt
> side currently?

Libvirt queries qemu capabilites via QMP. And in all places it can it 
preferes the latest recommended cmd line options (well, those known to a 
libvirt developer at the time he/she is writing the code). So as long as 
you remove only old stuff and libvirt refreshes itself when following 
best practicies we're okay.

> For example I don't see anything introspection related when we have been
> removing deprecated options recently.
>
> More exact question specific to this series usecase,
> how libvirt decides when to use -numa node,memdev or not currently?

It has a mechanism to tell if '-numa node,memdev=' is needed (i.e. there 
is no other way to satisfy user requested configuration) and only then 
it uses ,memdev. For all other cases it defaults to -numa node,mem= 
simply to keep backwards compatibility (as I'm explaining in another 
e-mail I've just sent to this list).

Anyway, in the libvirt code you want to be looking at:

src/qemu/qemu_command.c: qemuBuildNumaArgStr
src/qemu/qemu_command.c: qemuBuildMemoryCellBackendStr

Michal