Re: [Qemu-devel] [PATCH v2 4/7] scripts/qemu.py: set predefined machine type based on arch

[Eduardo Habkost]

On Wed, Oct 10, 2018 at 10:15:15AM -0400, Cleber Rosa wrote:
> 
> 
> On 10/10/18 9:59 AM, Cleber Rosa wrote:
> > 
> > 
> > On 10/10/18 9:46 AM, Eduardo Habkost wrote:
> >> On Wed, Oct 10, 2018 at 08:35:38AM -0400, Cleber Rosa wrote:
> >>>
> >>>
> >>> On 10/10/18 7:00 AM, Philippe Mathieu-Daudé wrote:
> >>>> On 10/10/2018 01:26, Cleber Rosa wrote:
> >>>>> Some targets require a machine type to be set, as there's no default
> >>>>> (aarch64 is one example).  To give a consistent interface to users of
> >>>>> this API, this changes set_machine() so that a predefined default can
> >>>>> be used, if one is not given.  The approach used is exactly the same
> >>>>> with the console device type.
> >>>>>
> >>>>> Also, even when there's a default machine type, for some purposes,
> >>>>> testing included, it's better if outside code is explicit about the
> >>>>> machine type, instead of relying on whatever is set internally.
> >>>>>
> >>>>> Signed-off-by: Cleber Rosa <address@hidden>
> >>>>> ---
> >>>>>  scripts/qemu.py | 22 +++++++++++++++++++++-
> >>>>>  1 file changed, 21 insertions(+), 1 deletion(-)
> >>>>>
> >>>>> diff --git a/scripts/qemu.py b/scripts/qemu.py
> >>>>> index d9e24a0c1a..fca9b76990 100644
> >>>>> --- a/scripts/qemu.py
> >>>>> +++ b/scripts/qemu.py
> >>>>> @@ -36,6 +36,15 @@ CONSOLE_DEV_TYPES = {
> >>>>>      r'^s390-ccw-virtio.*': 'sclpconsole',
> >>>>>      }
> >>>>>  
> >>>>> +#: Maps archictures to the preferred machine type
> >>>>> +MACHINE_TYPES = {
> >>>>> +    r'^aarch64$': 'virt',
> >>>>> +    r'^ppc$': 'g3beige',
> >>>>> +    r'^ppc64$': 'pseries',
> >>>>> +    r'^s390x$': 's390-ccw-virtio',
> >>>>> +    r'^x86_64$': 'q35',
> >>>>
> >>>> Why choose Q35 rather than PC (the default)?
> >>>>
> >>>> I was wondering about how to generate variants/machines.json but this is
> >>>> definitively something we want to do via a QMP query.
> >>>>
> >>>> Eduardo what do you think?
> >>>>
> >>>
> >>> It was motivated by Eduardo's initiative to make q35 the default "across
> >>> the board".  He can confirm and give more details.
> >>
> >> Making Q35 the default on applications using QEMU and libvirt is
> >> something I'd like to happen.  But I think the simplest way to do
> >> that is to change the QEMU default.  This way you won't need this
> >> table on qemu.py: you can just use the default provided by QEMU.
> >>
> > 
> > The idea is to bring consistency on how we're calling
> > "qemu-system-$(ARCH)", and at the same time apply the "explicit is
> > better than implicit" rule.
> > 
> > The most important fact is that some targets do not (currently) have
> > "the default provided by QEMU", aarch64 is one of them.
> > 
> > - Cleber.
> > 
> 
> So I ended up not relaying the question properly: should we default
> (even if explicitly adding "-machine") to "pc"?

I think using the default machine-type (when QEMU has a default)
would be less surprising for users of the qemu.py API.

Implicitly adding -machine when there's no default is also
surprising, but then it's a nice surprise: instead of crashing
you get a running VM.

Now, there are two other questions related to this:

If using 'pc' as default, should we always add -machine, or just
omit the machine-type name?  I think we should omit it unless the
caller asked for a specific machine-type name (because it would
be less surprising for users of the API).

About our default testing configuration for acceptance tests:
should acceptance tests run against PC by default?  Should it
test Q35?  Should we test both PC and Q35?  I'm not sure what's
the answer, but I think these decisions shouldn't affect the
qemu.py API at all.

-- 
Eduardo