Re: [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 <address@hidden> wrote:
>
> > On 1/15/20 5:52 PM, Igor Mammedov wrote:
> > > On Wed, 15 Jan 2020 16:34:53 +0100
> > > Peter Krempa <address@hidden> 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 <address@hidden>
> > > > > ---
> > > > > CC: address@hidden
> > > > > CC: address@hidden
> > > > > CC: address@hidden
> > > > > CC: address@hidden
> > > > > CC: address@hidden
> > > > > CC: address@hidden
> > > > > CC: address@hidden
> > > > > CC: address@hidden
> > > > > CC: address@hidden
> > > > > CC: address@hidden
> > > > > ---
> > > > >    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