linux-kernel - Re: [PATCH v2 01/24] x86/resctrl: Split struct rdt

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Date:   Wed, 31 Mar 2021 14:35:52 -0700
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     James Morse <james.morse@....com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Cc:     Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Babu Moger <Babu.Moger@....com>,
        shameerali.kolothum.thodi@...wei.com,
        Jamie Iles <jamie@...iainc.com>,
        D Scott Phillips OS <scott@...amperecomputing.com>
Subject: Re: [PATCH v2 01/24] x86/resctrl: Split struct rdt_resource

Hi James,

A significant time has passed since the first version and with that a 
lot of my context lost.

On 3/12/2021 9:58 AM, James Morse wrote:
> resctrl is the defacto Linux ABI for SoC resource partitioning features.
> To support it on another architecture, it needs to be abstracted from
> the features provided by Intel RDT and AMD PQoS, and moved to /fs/.
> 
> Start by splitting struct rdt_resource, (the name is kept to keep the noise
> down), and add some type-trickery to keep the foreach helpers working.

Could you please replace "add some type-trickery" with a description of 
the changes(tricks?) referred to? Comments in the code would be helpful 
also ... helping to avoid frowning at what at first glance seems like an 
out-of-bounds access.

> 
> Move everything that is particular to resctrl into a new header
> file, keeping the x86 hardware accessors where they are. resctrl code
> paths touching a 'hw' struct indicates where an abstraction is needed.

This establishes the significance of this patch. Here the rdt_resource 
struct is split up and it is this split that guides the subsequent 
abstraction. Considering this I find that this description does not 
explain the resulting split sufficiently.

Specifically, after reading the above summary I expect fs information in 
rdt_resource and hw information in rdt_hw_resource but that does not 
seem to be the case. For example, num_rmid is a property obtained from 
hardware but is found in rdt_resource while other hardware properties 
initialized at the same time are found in rdt_hw_resource. It is 
interesting to look at when the hardware is discovered (for example, 
functions like cache_alloc_hsw_probe(), __get_mem_config_intel(), 
__rdt_get_mem_config_amd(), rdt_get_cache_alloc_cfg()). Note how some of 
the discovered values end up in rdt_resource and some in 
rdt_hw_resource. I was expecting these properties discovered from 
hardware to be in rdt_hw_resource.

It is also not clear to me how these structures are intended to be used 
for related hardware properties. For example, rdt_resource keeps the 
properties alloc_capable/alloc_enabled/mon_capable/mon_enabled - but in 
this series companion properties of cdp_capable/cdp_enabled are 
introduced and placed in rdt_hw_resource. That seems contradicting to me.

Since this change is so foundational it would be very helpful if the 
resulting split could be explained in more detail.

Thank you

Reinette