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

[Thomas Huth <thuth@redhat.com>]

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

  Thomas