[libvirt] [PATCH v2 82/86] numa: forbid '-numa node, mem' for 5.0 and newer machine types

[Michal Privoznik]

On 1/16/20 1:37 PM, Igor Mammedov wrote:
> On Thu, 16 Jan 2020 11:42:09 +0100
> Michal Privoznik <mprivozn at redhat.com> wrote:
> 
>> On 1/15/20 5:52 PM, Igor Mammedov wrote:
>>> On Wed, 15 Jan 2020 16:34:53 +0100
>>> Peter Krempa <pkrempa at redhat.com> wrote:
>>>    
>>>> On Wed, Jan 15, 2020 at 16:07:37 +0100, Igor Mammedov wrote:
>>>>> Deprecation period is ran out and it's a time to flip the switch
>>>>> introduced by cd5ff8333a.
>>>>> Disable legacy option for new machine types and amend documentation.
>>>>>
>>>>> Signed-off-by: Igor Mammedov <imammedo at redhat.com>
>>>>> ---
>>>>> CC: peter.maydell at linaro.org
>>>>> CC: ehabkost at redhat.com
>>>>> CC: marcel.apfelbaum at gmail.com
>>>>> CC: mst at redhat.com
>>>>> CC: pbonzini at redhat.com
>>>>> CC: rth at twiddle.net
>>>>> CC: david at gibson.dropbear.id.au
>>>>> CC: libvir-list at redhat.com
>>>>> CC: qemu-arm at nongnu.org
>>>>> CC: qemu-ppc at nongnu.org
>>>>> ---
>>>>>    hw/arm/virt.c        |  2 +-
>>>>>    hw/core/numa.c       |  6 ++++++
>>>>>    hw/i386/pc.c         |  1 -
>>>>>    hw/i386/pc_piix.c    |  1 +
>>>>>    hw/i386/pc_q35.c     |  1 +
>>>>>    hw/ppc/spapr.c       |  2 +-
>>>>>    qemu-deprecated.texi | 16 ----------------
>>>>>    qemu-options.hx      |  8 ++++----
>>>>>    8 files changed, 14 insertions(+), 23 deletions(-)
>>>>
>>>> I'm afraid nobody bothered to fix it yet:
>>>>
>>>> https://bugzilla.redhat.com/show_bug.cgi?id=1783355
>>>
>>> It's time to start working on it :)
>>> (looks like just deprecating stuff isn't sufficient motivation,
>>> maybe actual switch flipping would work out better)
>>>    
>>
>> So how was the upgrade from older to newer version resolved? I mean, if
>> the old qemu used -numa node,mem=XXX and it is migrated to a host with
>> newer qemu, the cmd line can't be switched to -numa node,memdev=node0,
>> can it? I'm asking because I've just started working on this.
> 
> see commit cd5ff8333a3c87 for detailed info.
> Short answer is it's not really resolved [*],
> -numa node,mem will keep working on newer QEMU but only for old machine types
> new machine types will accept only -numa node,memdev.
> 
> One can check if "mem=' is supported by using QAPI query-machines
> and checking numa-mem-supported field. That field is flipped to false
> for 5.0 and later machine types in this patch.

Alright, so what we can do is the following:

1) For new machine types (pc-5.0/q35-5.0 and newer) use memdev= always.

2) For older machine types, we are stuck with mem= until qemu is capable 
of migrating from mem= to memdev=

I think this is a safe thing to do since migrating from one version of a 
machine type to another is not supported (since it can change guest 
ABI). And we will see how much 2) bothers us. Does this sound reasonable?

Michal