Re: [PULL 0/4] machine development tool

[Maksim Davydov]

On 3/6/24 04:57, Peter Xu wrote:

> On Tue, Mar 05, 2024 at 03:43:41PM +0100, Markus Armbruster wrote:
> > Peter Maydell <peter.maydell@linaro.org> writes:
> >
> > > On Mon, 4 Mar 2024 at 13:52, Maksim Davydov <davydov-max@yandex-team.ru> wrote:
> > > > The following changes since commit e1007b6bab5cf97705bf4f2aaec1f607787355b8:
> > > >
> > > >   Merge tag 'pull-request-2024-03-01' of https://gitlab.com/thuth/qemu into staging (2024-03-01 10:14:32 +0000)
> > > >
> > > > are available in the Git repository at:
> > > >
> > > >   https://gitlab.com/davydov-max/qemu.git tags/pull-compare-mt-2024-03-04
> > > >
> > > > for you to fetch changes up to 7693a2e8518811a907d73a85807ee71dac8fabcb:
> > > >
> > > >   scripts: add script to compare compatibility properties (2024-03-04 14:10:53 +0300)
> > > >
> > > > ----------------------------------------------------------------
> > > > Please note. This is the first pull request from me.
> > > > My public GPG key is available here
> > > > https://keys.openpgp.org/vks/v1/by-fingerprint/CDB5BEEF8837142579F5CDFE8E927E10F72F78D4
> > > >
> > > > ----------------------------------------------------------------
> > > > scripts: add a new script for machine development
> > > >
> > > > ----------------------------------------------------------------
> > > >           
> > > Hi; I would prefer this to go through some existing submaintainer
> > > tree if possible, please.
> > >         
> > Migration?  QOM?  Not sure.  Cc'ing the maintainers anyway.
> >       
> Yeah this seems like migration relevant.. however now I'm slightly confused
> on when this script should be useful.
>
> According to:
>
> https://lore.kernel.org/qemu-devel/20240222153912.646053-5-davydov-max@yandex-team.ru/
>
>         This script runs QEMU to obtain compat_props of machines and
>         default values of different types of drivers to produce comparison
>         table. This table can be used to compare machine types to choose
>         the most suitable machine or compare binaries to be sure that
>         migration to the newer version will save all device
>         properties. Also the json or csv format of this table can be used
>         to check does a new machine affect the previous ones by comparing
>         tables with and without the new machine.
>
> In regards to "choose the most suitable machine": why do you need to choose
> a machine?
>
> If it's pretty standalone setup, shouldn't we always try to use the latest
> machine type if possible (as normally compat props are only used to keep
> compatible with old machine types, and the default should always be
> preferred). If it's a cluster setup, IMHO it should depend on the oldest
> QEMU version that plans to be supported.  I don't see how such comparison
> helps yet in either of the cases..
>
> In regards to "compare binaries to be sure that migration to the newer
> version will save all device properties": do we even support migrating
> _between_ machine types??
>
> Sololy relying on compat properties to detect device compatibility is also
> not reliable.  For example, see VMStateField.field_exists() or similarly,
> VMStateDescription.needed(), which are hooks that device can provide to
> dynamically decide what device state to be saved/loaded.  Such things are
> not reflected in compat properties, so even if compat properties of all
> devices are the same between two machine types, it may not mean that the
> migration stream will always be compatible.
>
> Thanks,
>

In fact, the last commit describes the meaning of this series best. Perhaps
it should have been moved to the cover letter:
Often, many teams have their own "machines" inherited from "upstream" to
manage default values of devices. This is done because of the limitations
imposed by the compatibility requirements or the desire to help their
customers better configure their devices. And since machine type has
a hard-to-read structure, it is very easy to make a mistake when transferring
default values from one machine to another. For example, when some property
is set for the entire abstract class x86_64-cpu (which will be applied to all
models), and then rolled back for a specific model. The situation is about
the same with changing the default values of devices: if the value changes
in the description of the device itself, then you need to make sure that
nothing changes when using the current machine.
Therefore, there was a desire to make a dev tool that will help quickly expand
the definition of a machine or compare several machines from different binary
files. It can be used when rebasing to a new version of qemu and/or for
automatic tests.

-- 
Best regards,
Maksim Davydov