Re: [PATCH for-7.0] i386: Deprecate the -no-hpet QEMU command line option

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

On Mon, Dec 06, 2021 at 10:44:50AM +0100, Thomas Huth wrote:
> On 06/12/2021 10.19, Peter Krempa wrote:
> > On Mon, Dec 06, 2021 at 10:02:44 +0100, Thomas Huth wrote:
> > > On 06/12/2021 09.57, Peter Krempa wrote:
> > > > On Mon, Dec 06, 2021 at 09:47:58 +0100, Thomas Huth wrote:
> > > > > On 06/12/2021 09.40, Thomas Huth wrote:
> > > > > > The HPET setting has been turned into a machine property a while ago
> > > > > > already, so we should finally do the next step and deprecate the
> > > > > > legacy CLI option, too.
> > > > > > While we're at it, add a proper help text for the machine property, too.
> > > > > > 
> > > > > > Signed-off-by: Thomas Huth <thuth@redhat.com>
> > > > > > ---
> > > > > >     docs/about/deprecated.rst | 6 ++++++
> > > > > >     hw/i386/pc.c              | 2 ++
> > > > > >     qemu-options.hx           | 2 +-
> > > > > >     softmmu/vl.c              | 1 +
> > > > > >     4 files changed, 10 insertions(+), 1 deletion(-)
> > > > > > 
> > > > > > diff --git a/docs/about/deprecated.rst b/docs/about/deprecated.rst
> > > > > > index 5693abb663..1dfe69aa6a 100644
> > > > > > --- a/docs/about/deprecated.rst
> > > > > > +++ b/docs/about/deprecated.rst
> > > > > > @@ -198,6 +198,12 @@ form is preferred.
> > > > > >     Using ``-drive if=none`` to configure the OTP device of the sifive_u
> > > > > >     RISC-V machine is deprecated. Use ``-drive if=pflash`` instead.
> > > > > > +``-no-hpet`` (since 7.0)
> > > > > > +''''''''''''''''''''''''
> > > > > > +
> > > > > > +The HPET setting has been turned into a machine property.
> > > > > > +Use ``-machine hpet=off`` instead.
> > > > > [...]
> > > > > 
> > > > > Forgot to CC: the libvirt folks, doing so now.
> > > > > 
> > > > > Seems like libvirt is still using -no-hpet in some few spots, so I guess
> > > > > these would need to be changed first, before we could finally remove this
> > > > > option in QEMU?
> > > > 
> > > > Yes we need to switch to the new property first.
> > > > 
> > > > Is the new way via -machine property by any chance usable with
> > > > qemu-2.11? If yes, then we can do it unconditionally, otherwise we'll
> > > > need a witness to detect the support for the new flag as a qemu
> > > > capability.
> > > 
> > > The machine property has been added just a year ago:
> > > 
> > >   https://gitlab.com/qemu-project/qemu/-/commit/0259c78ca79
> > > 
> > > So it's just available on QEMU v5.2.0 and newer.
> > 
> > Okay, so we can't unfortunately always use the new way.
> > 
> > I had a brief look in what libvirt queries to build the capability list
> > and unfortunately neither query-command-line-options nor query-machines
> > list anything which we could detect.
> > 
> > Since only the config knob is changing the presence in qom-list-types
> > doesn't help either.
> > 
> > So if qemu want's to deprecate the '-no-hpet' spelling we need something
> > which we can detect by one of the above means or other probe command to
> > use the new spelling.
> 
> I just had another chat with Peter on IRC, and seems like it could be
> detected by running "qom-list-properties" on e.g. the "generic-pc-machine"
> object. However, libvirt does not have that information in their test data
> yet, so it's not a very trivial change to support this in libvirt.

It is mostly just a tedious matter of getting sample QMP replies for
the qom-list-properties command on various older QEMU versions to
make sure our test suite does the right thing.

> Thus, please don't merge this patch yet, it will currently cause more hassle
> than benefit.

I don't see any problem in QEMU going ahead with this. If it is deprecated
in 7.0, then it won't be until 7.2 that its deleted, which is Dec 2022
timeframe. A year is enough time for libvirt to adapt.

Regards,
Daniel
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