[libvirt PATCH v2 2/3] conf: support firmware ISA debug console

[Daniel P. Berrangé]

On Wed, Feb 02, 2022 at 06:46:25AM -0800, Andrea Bolognani wrote:
> On Wed, Feb 02, 2022 at 02:32:32PM +0000, Daniel P. Berrangé wrote:
> > On Wed, Feb 02, 2022 at 06:29:32AM -0800, Andrea Bolognani wrote:
> > > On Wed, Feb 02, 2022 at 12:44:44PM +0000, Daniel P. Berrangé wrote:
> > > > Introduce support for
> > > >
> > > >   <serial type='pty'>
> > > >     <target type='isa-debug'>
> > > >       <model type='isa-debugcon'/>
> > > >     </target>
> > > >     <address type='isa' iobase='0x402'/>
> > > >   </console>
> > > >
> > > > which is used as a way to receive debug messages from the
> > > > firmware on x86 platforms.
> > > >
> > > > Note that the default port is 0x0xe9 since that's the original
> > > > Bochs debug port. Thus for use with SeaBIOS/OVMF, the iobase
> > > > port needs to be explicitly set to 0x402.
> > >
> > > The default is not set anywhere in the code, so I guess we're relying
> > > on the fact that QEMU has that as the current default. Wouldn't it
> > > make sense to have it reflected in the XML as well?
> > >
> > > The previous version used 0x402 as the default, which apparently
> > > doesn't match QEMU's own but is what most people will want in
> > > practice. Wouldn't that be a better default for us? People who care
> > > about Bochs compatibility can always explicitly set the appropriate
> > > value themselves.
> >
> > The previous bversion was a mistake because I incorrectly thought
> > 402 was QEMU's own default. I also mistakenly thought we needed
> > to set it explicitly for ABI compat, but that's not the case because
> > the machine type ensures that.
> >
> > So preferrable to not override QEMU's own defaults in libvirt.
> 
> I think it would still make sense to reflect QEMU's current default
> in the XML, which would make sure that the same input XML results in
> the same device even if QEMU decides to change its defaults later on.

We don't do that for any existing ISA devices AFAICT, so I'm not finding
a compelling reason to treat isa-debugcon as special.


Regards,
Daniel
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