Re: [PATCH v8 00/21] Introduce smp.modules for x86 in QEMU

[Daniel P . Berrangé]

On Thu, Feb 01, 2024 at 10:57:32AM +0800, Zhao Liu wrote:
> Hi Daniel,
> 
> On Wed, Jan 31, 2024 at 10:28:42AM +0000, Daniel P. Berrangé wrote:
> > Date: Wed, 31 Jan 2024 10:28:42 +0000
> > From: "Daniel P. Berrangé" <berrange@redhat.com>
> > Subject: Re: [PATCH v8 00/21] Introduce smp.modules for x86 in QEMU
> > 
> > On Wed, Jan 31, 2024 at 06:13:29PM +0800, Zhao Liu wrote:
> > > From: Zhao Liu <zhao1.liu@intel.com>
> 
> [snip]
> 
> > > However, after digging deeper into the description and use cases of
> > > cluster in the device tree [3], I realized that the essential
> > > difference between clusters and modules is that cluster is an extremely
> > > abstract concept:
> > >   * Cluster supports nesting though currently QEMU doesn't support
> > >     nested cluster topology. However, modules will not support nesting.
> > >   * Also due to nesting, there is great flexibility in sharing resources
> > >     on clusters, rather than narrowing cluster down to sharing L2 (and
> > >     L3 tags) as the lowest topology level that contains cores.
> > >   * Flexible nesting of cluster allows it to correspond to any level
> > >     between the x86 package and core.
> > > 
> > > Based on the above considerations, and in order to eliminate the naming
> > > confusion caused by the mapping between general cluster and x86 module
> > > in v7, we now formally introduce smp.modules as the new topology level.
> > 
> > What is the Linux kernel calling this topology level on x86 ?
> > It will be pretty unfortunate if Linux and QEMU end up with
> > different names for the same topology level.
> > 
> 
> Now Intel's engineers in the Linux kernel are starting to use "module"
> to refer to this layer of topology [4] to avoid confusion, where
> previously the scheduler developers referred to the share L2 hierarchy
> collectively as "cluster".
> 
> Looking at it this way, it makes more sense for QEMU to use the
> "module" for x86.

I was thinking specificially about what Linux calls this topology when
exposing it in sysfs and /proc/cpuinfo. AFAICT, it looks like it is
called 'clusters' in this context, and so this is the terminology that
applications and users are going to expect.

I think it would be a bad idea for QEMU to diverge from this and call
it modules.

With regards,
Daniel
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