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

From: Michal Privoznik
Subject: Re: [Qemu-ppc] [Qemu-devel] [libvirt] [PATCH 1/2] numa: deprecate 'mem' parameter of '-numa node' option
Date: Mon, 4 Mar 2019 15:34:06 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.5.2

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
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

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

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


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