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

[Zhao Liu <zhao1.liu@linux.intel.com>]

On Wed, Feb 21, 2024 at 01:41:35PM +0100, Markus Armbruster wrote:
> Date: Wed, 21 Feb 2024 13:41:35 +0100
> From: Markus Armbruster <armbru@redhat.com>
> Subject: Re: [PATCH v8 00/21] Introduce smp.modules for x86 in QEMU
> 
> Zhao Liu <zhao1.liu@linux.intel.com> writes:
> 
> > From: Zhao Liu <zhao1.liu@intel.com>
> >
> > Hi list,
> >
> > This is the our v8 patch series, rebased on the master branch at the
> > commit 11be70677c70 ("Merge tag 'pull-vfio-20240129' of
> > https://github.com/legoater/qemu into staging").
> >
> > Compared with v7 [1], v8 mainly has the following changes:
> >   * Introduced smp.modules for x86 instead of reusing current
> >     smp.clusters.
> >   * Reworte the CPUID[0x1F] encoding.
> >
> > Given the code change, I dropped the most previously gotten tags
> > (Acked-by/Reviewed-by/Tested-by from Michael & Babu, thanks for your
> > previous reviews and tests!) in v8.
> >
> > With the description of the new modules added to x86 arch code in v7 [1]
> > cover letter, the following sections are mainly the description of
> > the newly added smp.modules (since v8) as supplement.
> >
> > Welcome your comments!
> >
> >
> > Why We Need a New CPU Topology Level
> > ====================================
> >
> > For the discussion in v7 about whether we should reuse current
> > smp.clusters for x86 module, the core point is what's the essential
> > differences between x86 module and general cluster.
> >
> > Since, cluster (for ARM/riscv) lacks a comprehensive and rigorous
> > hardware definition, and judging from the description of smp.clusters
> > [2] when it was introduced by QEMU, x86 module is very similar to
> > general smp.clusters: they are all a layer above existing core level
> > to organize the physical cores and share L2 cache.
> >
> > 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.
> >
> >
> > Where to Place Module in Existing Topology Levels
> > =================================================
> >
> > The module is, in existing hardware practice, the lowest layer that
> > contains the core, while the cluster is able to have a higher topological
> > scope than the module due to its nesting.
> >
> > Thereby, we place the module between the cluster and the core, viz:
> >
> >     drawer/book/socket/die/cluster/module/core/thread
> >
> >
> > Additional Consideration on CPU Topology
> > ========================================
> >
> > Beyond this patchset, nowadays, different arches have different topology
> > requirements, and maintaining arch-agnostic general topology in SMP
> > becomes to be an increasingly difficult thing due to differences in
> > sharing resources and special flexibility (e.g., nesting):
> >   * It becomes difficult to put together all CPU topology hierarchies of
> >     different arches to define complete topology order.
> >   * It also becomes complex to ensure the correctness of the topology
> >     calculations.
> >       - Now the max_cpus is calculated by multiplying all topology
> >         levels, and too many topology levels can easily cause omissions.
> >
> > Maybe we should consider implementing arch-specfic topology hierarchies.
> >
> >
> > [1]: https://lore.kernel.org/qemu-devel/20240108082727.420817-1-zhao1.liu@linux.intel.com/
> > [2]: https://lists.gnu.org/archive/html/qemu-devel/2023-02/msg04051.html
> > [3]: https://www.kernel.org/doc/Documentation/devicetree/bindings/cpu/cpu-topology.txt
> 
> Have you considered putting an abridged version of your lovely rationale
> into a commit message, so it can be found later more easily?
>

Sure, I'll. Thanks for helping me improve my commit message. ;-)

Regards,
Zhao