Re: [Qemu-devel] [PATCH] pc: acpi: force FADT rev1 for old i440fx machine types

[Daniel P. Berrange]

On Fri, Jul 21, 2017 at 12:10:48PM +0200, Igor Mammedov wrote:
> On Fri, 21 Jul 2017 10:49:55 +0100
> "Daniel P. Berrange" <address@hidden> wrote:
> 
> > On Fri, Jul 21, 2017 at 11:32:11AM +0200, Igor Mammedov wrote:
> > > w2k used to boot on QEMU until we bumped revision of FADT to rev3
> > > (commit 77af8a2b hw/i386: Use Rev3 FADT (ACPI 2.0) instead of Rev1 to 
> > > improve guest OS support.)
> > > 
> > > Considering that w2k is ancient and long time EOLed, leave default
> > > rev3 but make pc-i440fx-2.9 and older machine types to force rev1
> > > so old setups won't break (w2k could boot).  
> > 
> > There needs to be a machine type property added to control this
> > feature. When provisioning new VMs, management apps need to be
> > able to set the property explicitly - having them rely on picking
> > particular machine type name+versions is not viable, because
> > downstream vendors replace the machine types with their own
> > names + versions.
> having property doesn't really help here and we don't do it for every
> compat tweak /ex: save_tsc_khz, linuxboot_dma_enabled/.

If those compat tweaks affect compatibility with particular guest
OS then they should definitely be exposed as properties too.

> Management would not benefit much from having property vs machine version
> as it would have to encode somewhere that for w2k it should set
> some machine property or pick a particular machine type.

It *would* be a significant benefit - property names are stable, machine
type versions are not stable becasue downstream vendors change them.

> Also with new machine type deprecation policy we would be able
> easily to phase out rev1 support along with 2.9 machine,
> but if you expose property then removing it would break
> CLI not only for 2.9 but possible later machines if it's set there.

We have the freedom to deprecate properties too if they become a significant
burden.  If removing machine types prevents us running certain guest OS,
because we don't have a property to override, that would be a mark against
removing the machine types at all IMHO. 

> So I'm against adding properties/CLI options for unless we have to in this
> case, and I'm not convinced that w2k deserves it.

w2k is just one OS that we happen to know of that breaks - who knows how
many others suffer the same fate. So making decisions based on whether
you care about a specific OS is flawed IMHO.

Regards,
Daniel
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