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

[Markus Armbruster]

Zhao Liu <zhao1.liu@intel.com> writes:

> 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?

Good questions.  However, before we head deeper into the weeds here, I
feel we should discuss the things below.  And before we do that, I need
a clear understanding of the use case.  Elsewhere in this thread, I just
described the use case as I understand it.  Please reply there.  I'll
then come back to this message.

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