Specifying Cache Granule Size in Platform


Aaron Durbin
 

Hi All,

During the Platform HSC the topic of specifying an expected cache granule size for a platform was brought up. Below are some thoughts/observations on the topic. The purpose of this email is to provide a basis for discussion. 

Philip indicated cbo.zero would be the main target for specifying the size. If we went down that path, I think it'd be important to also specify the size for cbo.flush as well.

If the granule size is only determined at run time there inherently are solutions that need to be constructed to deal with a number of scenarios. Some cases to think about:

1. Userland drivers
2. VM migration
3. object layout in memory for optimization purposes
4. inconsistent granule size within different harts in the system

Ved brought up how it's important to interrogate the cache hierarchy for these attributes, and that it's important to know on a per-hart basis what cache granule size it deals with.

We can certainly engineer solutions for the various cases in either scenario, but I think the discussion around specifying the granule size should be in understanding how we'd deal with those solutions going forward. And please offer any other scenarios that may come to mind. Lastly, I apologize if I missed any particular commentary on this subject. 

Thanks.

-Aaron

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