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

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Jan 31, 2024 at 06:13:29PM +0800, Zhao Liu wrote:
> 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.

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.

With regards,
Daniel
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