Re: [Qemu-devel] [PATCH] pc: reduce duplication, fix PIIX descriptions

[Eduardo Habkost]

On Tue, Aug 27, 2013 at 09:51:22AM +0200, Markus Armbruster wrote:
> "Michael S. Tsirkin" <address@hidden> writes:
> 
> > We have a lot of code duplication between machine types,
> > this increases with each new machine type
> > and each new field.
> >
> > This has already introduced a minor bug: description
> > for pc-1.3 says "Standard PC" while description for
> > pc-1.4 is "Standard PC (i440FX + PIIX, 1996)"
> > which makes you think 1.3 is somehow more standard,
> > or newer, while in fact it's a revision of the same PC.
> 
> I wouldn't call it a bug.  We're in the habit of never changing old
> machine types, so when we created pc-i440fx-1.4 and pc-q35-1.4 with a
> .desc that clearly explains the difference, we didn't change the older
> versions the PC machine types as well.
> 
> That may have been overly cautious.  As long as .desc is not exposed to
> the guest, it's not ABI, thus can be changed.

I just confirmed that on qemu-system-x86_64 the only place where .desc
is used is on the machine_parse() help/error code at vl.c. I don't know
about other architectures.

I suggest we explicitly document the field as a human-readable
machine-type description that should be never exposed to the guest (just
in case somebody decides to use .desc on some guest-visible table one
day).

-- 
Eduardo