Re: [PATCH 8/8] qemu-options: Add the description of smp-cache object

[Zhao Liu]

Hi Markus and Daniel,

I have the questions about the -object per cache implementation:

On Wed, Jul 24, 2024 at 02:39:29PM +0200, Markus Armbruster wrote:
> Date: Wed, 24 Jul 2024 14:39:29 +0200
> From: Markus Armbruster <armbru@redhat.com>
> Subject: Re: [PATCH 8/8] qemu-options: Add the description of smp-cache
>  object
> 
> Zhao Liu <zhao1.liu@intel.com> writes:
> 
> > Hi Markus,
> >
> > On Mon, Jul 22, 2024 at 03:37:43PM +0200, Markus Armbruster wrote:
> >> Date: Mon, 22 Jul 2024 15:37:43 +0200
> >> From: Markus Armbruster <armbru@redhat.com>
> >> Subject: Re: [PATCH 8/8] qemu-options: Add the description of smp-cache
> >>  object
> >> 
> >> Zhao Liu <zhao1.liu@intel.com> writes:
> >> 
> >> > Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
> >> 
> >> This patch is just documentation.  The code got added in some previous
> >> patch.  Would it make sense to squash this patch into that previous
> >> patch?
> >
> > OK, I'll merge them.
> >
> >> > ---
> >> > Changes since RFC v2:
> >> >  * Rewrote the document of smp-cache object.
> >> >
> >> > Changes since RFC v1:
> >> >  * Use "*_cache=topo_level" as -smp example as the original "level"
> >> >    term for a cache has a totally different meaning. (Jonathan)
> >> > ---
> >> >  qemu-options.hx | 58 +++++++++++++++++++++++++++++++++++++++++++++++++
> >> >  1 file changed, 58 insertions(+)
> >> >
> >> > diff --git a/qemu-options.hx b/qemu-options.hx
> >> > index 8ca7f34ef0c8..4b84f4508a6e 100644
> >> > --- a/qemu-options.hx
> >> > +++ b/qemu-options.hx
> >> > @@ -159,6 +159,15 @@ SRST
> >> >          ::
> >> >  
> >> >              -machine 
> >> > cxl-fmw.0.targets.0=cxl.0,cxl-fmw.0.targets.1=cxl.1,cxl-fmw.0.size=128G,cxl-fmw.0.interleave-granularity=512
> >> > +
> >> > +    ``smp-cache='id'``
> >> > +        Allows to configure cache property (now only the cache topology 
> >> > level).
> >> > +
> >> > +        For example:
> >> > +        ::
> >> > +
> >> > +            -object 
> >> > '{"qom-type":"smp-cache","id":"cache","caches":[{"name":"l1d","topo":"core"},{"name":"l1i","topo":"core"},{"name":"l2","topo":"module"},{"name":"l3","topo":"die"}]}'
> >> > +            -machine smp-cache=cache
> >> >  ERST
> >> >  
> >> >  DEF("M", HAS_ARG, QEMU_OPTION_M,
> >> > @@ -5871,6 +5880,55 @@ SRST
> >> >          ::
> >> >  
> >> >              (qemu) qom-set /objects/iothread1 poll-max-ns 100000
> >> > +
> >> > +    ``-object 
> >> > '{"qom-type":"smp-cache","id":id,"caches":[{"name":cache_name,"topo":cache_topo}]}'``
> >> > +        Create an smp-cache object that configures machine's cache
> >> > +        property. Currently, cache property only include cache topology
> >> > +        level.
> >> > +
> >> > +        This option must be written in JSON format to support JSON list.
> >> 
> >> Why?
> >
> > I'm not familiar with this, so I hope you could educate me if I'm wrong.
> >
> > All I know so far is for -object that defining a list can only be done in
> > JSON format and not with a numeric index like a keyval based option, like:
> >
> > -object smp-cache,id=cache0,caches.0.name=l1i,caches.0.topo=core: Parameter 
> > 'caches' is missing
> >
> > the above doesn't work.
> >
> > Is there any other way to specify a list in command line?
> 
> The command line is a big, sprawling mess :)
> 
> -object supports either a JSON or a QemuOpts argument.  *Not* keyval!
> 
> Both QemuOpts and keyval parse something like KEY=VALUE,...  Keyval
> supports arrays and objects via dotted keys.  QemuOpts doesn't natively
> support arrays and objects, but its users can hack around that
> limitation in various ways.  -object doesn't.  So you're right, it's
> JSON or bust here.
> 
> However, if we used one object per cache instead, we could get something
> like
> 
>     -object smp-cache,name=l1d,...
>     -object smp-cache,name=l1u,...
>     -object smp-cache,name=l2,...
>     ...

Current, I use -object to create a smp_cache object, and link it to
MachineState by -machine,smp-cache=obj_id.

Then for the objects per cache, how could I link them to machine?

Is it possible that I create something static in smp_cache.c and expose
all the cache information to machine through some interface?

Additionally, I would like to consider for the long term heterogeneous
cache, as asked before in [1], does the object per cache conflict with
the cache device I'm considering? Considering cache device is further
because I want to create CPU/cache topology via -device and build a
topology tree.

[1]: https://lore.kernel.org/qemu-devel/Zl88DYwLE3ScDF5F@intel.com/

I think this is becoming a nightmare I can't get around. Naming is
difficult, and sorting out interface design I think is also a difficult
task.

If you feel that there is indeed a conflict, then I'm also willing
to fall back to -smp again and do it based on keyval's list, as originally
suggested by Daniel. Sorry for the repetition on thoughts/design, I hope
that discussion with you I can make sense of the current and subsequent
paths without getting out of hand!

Best Regards,
Zhao