Re: [PATCH for-9.0] docs/about: Mark the iaspc machine type as deprecated

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Mar 27, 2024 at 02:09:17PM +0100, Igor Mammedov wrote:
> On Wed, 27 Mar 2024 10:49:43 +0000
> Daniel P. Berrangé <berrange@redhat.com> wrote:
> 
> > On Tue, Mar 26, 2024 at 05:16:32PM +0100, Igor Mammedov wrote:
> > > On Tue, 26 Mar 2024 14:29:58 +0100
> > > Philippe Mathieu-Daudé <philmd@linaro.org> wrote:
> > >   
> > > > Hi Igor,
> > > > 
> > > > On 26/3/24 14:08, Thomas Huth wrote:  
> > > > > 
> > > > > s/iaspc/isapc/ in the subject
> > > > > 
> > > > > On 26/03/2024 13.51, Igor Mammedov wrote:    
> > > > >> ISAPC machine was introduced 25 years ago and it's a lot of time since
> > > > >> such machine was around with real ISA only PC hardware practically 
> > > > >> defunct.
> > > > >> Also it's slowly bit-rots (for example: I was able to boot RHEL6 on 
> > > > >> RHEL9 host
> > > > >> in only TCG mode, while in KVM mode it hung in the middle of boot)    
> > > > 
> > > > I'm quite opposed to this patch. QEMU models various very-old /
> > > > defunct hardware. I'm pretty sure Bernhard and myself are OK to
> > > > keep maintaining it, besides we are working in separating it from
> > > > the i440fx+piix machine. Also, this machine is particularly
> > > > interesting for my single-binary experiments.  
> > > 
> > > it would not be fair to ask you or Bernard to deal with every
> > > case where ISAPC related code gets in a way, nor it's fair to
> > > ask other contributors to ensure that their patches don't break
> > > semi-working ISAPC or refactor code that relates to it. 
> > > 
> > > [
> > > for example I'd like to refactor smbios parts in the image
> > > ACPI table builder, but the I'd have to do it for legacy
> > > part as well without means to verify that. Sure it can be
> > > done but at cost of extra time spent to rewrite something
> > > that would never be used and to find test env to verify
> > > touched code.
> > > ]  
> > 
> > Is SMBIOS even relevant for isapc ? IIUC, the first SMBIOS spec
> > is from 1999, while PCI has been around since 1992.
> 
> Theoretically SMBIOS can still be used with isapc,
> (that's how I've tested factoring out legacy part by running
> RHEL6 in TCG mode) 
> Whether it's used in practice somewhere else is unknown. 
> 
> > IOW, we shouldn't even be exposing SMBIOS with the isapc
> > machine type. If we address that, then isapc has no impact
> > on your ability to refactor SMBIOS code.
> 
> It's question of whether we are willing to do unthinkable,
> i.e. to break QEMU <-> guest ABI for isapc case by removing
> corresponding fwcfg entries.

There has never been any ABI stability requirement for 'isapc'
as it is not a versioned machine type.

> With migration ignored it shouldn't be a problem.
> Question is: does anyone care about migration with isapc?
> If not, I'd gladly axe smbios legacy parts in 9.1

Migration is irrelevant unless someone steps forward to
commit to long term versioning of the machine type.

> But given how easy one can spawn old qemu environment to

There are ways to get access to an old QEMU, but I would
not call them easy when compared to using the QEMU that
is shipped as standard by your distro.

For a while a tried to maintain an archive of old QEMU
versions built for modern distros and it was a surprisingly
painful exercise. We tend to forget just how often QEMU's
build gets broken by new compilers, or new glibc system
header changes, or other 3rd party packages. 

Telling users to find an old QEMU is not an attractive
proposition, except as a last resort.

With regards,
Daniel
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