linux-kernel - RE: [RFC PATCH 0/7] Add driver registration i/f to resctrl

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Message-ID: <SJ1PR11MB6083031582F2A61A7D24ABAFFC749@SJ1PR11MB6083.namprd11.prod.outlook.com>
Date:   Thu, 11 May 2023 20:35:21 +0000
From:   "Luck, Tony" <tony.luck@...el.com>
To:     "Chatre, Reinette" <reinette.chatre@...el.com>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Shaopeng Tan <tan.shaopeng@...itsu.com>,
        Jamie Iles <quic_jiles@...cinc.com>,
        James Morse <james.morse@....com>,
        Babu Moger <babu.moger@....com>
CC:     "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [RFC PATCH 0/7] Add driver registration i/f to resctrl

Reinette,

You asked for examples of resources that don't fit well into the existing schemata model.

Here's the details behind one of those cases.

Existing resources have a single value per domain (e.g. a cache-way bitmask, or a memory
bandwidth percentage or target). One of my new resources has several parameters. At first
glance this might be solved by just listing a comma separated list of these parameters for
each domain. So the schemata entry for feature XYZ that has two domains might look like
this:

XYZ:0=param1,param2,param3;1=param1,param2,param3

But this feature has a second problem. The hardware supports a very limited set of variations.
This could be handled by reporting num_closid for this resource to that low number. But then
resctrl core code would limit all resources to that value. Instead the h/w allows programming
a mapping feature from closid numbers to resource instances (as the saying goes "any computer
science problem can be solved with one extra level of indirection").

So if the driver named these instances: A, B, C, D. Then a schemata file might look like:

XYZ:0=B;1=C

meaning that the driver will set up so CLOSID for this resctrl resource is mapped to instance "B"
on domain 0 and to instance "C" on domain 1.

The driver provides a way to set param1, param2, param3 for each of the A, B, C, D
instances.

-Tony