Re: [PATCH v3 6/7] i386/pc: Support cache topology in -machine for PC machine

[Daniel P . Berrangé]

On Sat, Oct 12, 2024 at 06:44:28PM +0800, Zhao Liu wrote:
> Allow user to configure l1d, l1i, l2 and l3 cache topologies for PC
> machine.
> 
> Additionally, add the document of "-machine smp-cache" in
> qemu-options.hx.
> 
> Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
> Tested-by: Yongwei Ma <yongwei.ma@intel.com>
> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> ---
> Changes since Patch v2:
>  * Polished the document. (Jonathan)
> 
> Changes since Patch v1:
>  * Merged document into this patch. (Markus)
> 
> Changes since RFC v2:
>  * Used cache_supported array.
> ---
>  hw/i386/pc.c    |  4 ++++
>  qemu-options.hx | 26 +++++++++++++++++++++++++-
>  2 files changed, 29 insertions(+), 1 deletion(-)
> 
>              -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.0.cache=cachename,smp-cache.0.topology=topologylevel``
> +        Define cache properties for SMP system.
> +
> +        ``cache=cachename`` specifies the cache that the properties will be
> +        applied on. This field is the combination of cache level and cache
> +        type. It supports ``l1d`` (L1 data cache), ``l1i`` (L1 instruction
> +        cache), ``l2`` (L2 unified cache) and ``l3`` (L3 unified cache).
> +
> +        ``topology=topologylevel`` sets the cache topology level. It accepts
> +        CPU topology levels including ``thread``, ``core``, ``module``,
> +        ``cluster``, ``die``, ``socket``, ``book``, ``drawer`` and a special
> +        value ``default``. If ``default`` is set, then the cache topology 
> will
> +        follow the architecture's default cache topology model. If another
> +        topology level is set, the cache will be shared at corresponding CPU
> +        topology level. For example, ``topology=core`` makes the cache shared
> +        by all threads within a core.
> +
> +        Example:
> +
> +        ::
> +
> +            -machine 
> smp-cache.0.cache=l1d,smp-cache.0.topology=core,smp-cache.1.cache=l1i,smp-cache.1.topology=core

There are 4 cache types, l1d, l1i, l2, l3.

In this example you've only set properties for l1d, l1i caches.

What does this mean for l2 / l3 caches ?

Are they reported as not existing, or are they to be reported at
some built-in default topology level. If the latter, how does the
user know what that built-in default is, and avoid nonsense like
l1d being at socket level, and l3 being at the core level ? Can
we explicitly disable a l2/l3 cache, or must it always exists ?

The QAPI has an "invalid" topology level. You've not documented
that as permitted here, but the qapi parser will happily allow
it. What semantics will that have ? 

With regards,
Daniel
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