linux-kernel - Re: [PATCH 1/2] x86/resctrl: Add io_alloc_min_cbm

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Message-ID: <53iuqiul6uo7zj6sfckm2h465cjusaxvpxrtdmnbmhvbzuqiq6@7ogm3zqddzjd>
Date: Fri, 7 Nov 2025 17:25:09 -0500
From: Aaron Tomlin <atomlin@...mlin.com>
To: "Moger, Babu" <bmoger@....com>
Cc: tony.luck@...el.com, reinette.chatre@...el.com, Dave.Martin@....com, 
	james.morse@....com, babu.moger@....com, tglx@...utronix.de, mingo@...hat.com, 
	bp@...en8.de, dave.hansen@...ux.intel.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] x86/resctrl: Add io_alloc_min_cbm_all interface for
 CBM reset

On Fri, Nov 07, 2025 at 04:02:39PM -0600, Moger, Babu wrote:
> Hi Aaron,
> 
> Thanks for trying the patches.

Hi Babu,

Thank you very much for taking the time to review the patch and for
pointing out the efficiency of the existing interface. You are correct that
option [1], echo "0=0;1=0" > io_alloc_cbm, is also accomplished with a
single system call.

However, the primary justification for introducing "io_alloc_min_cbm_all"
lies in abstraction and the knowledge burden placed upon the user, not the
system call count.

To successfully execute option [1], the user is required to have advanced
knowledge of the system's current cache topology and capabilities:

    Domain ID Knowledge: The user must know every active Domain ID (e.g.,
                         0, 1, 2, etc.) that supports the resource and
                         manually enumerate them in the write string. If a
                         domain is missed, it will not be reset.

    Minimal CBM Knowledge: The user must know the minimal supported number
                           of consecutive bits that constitute a "cleared"
                           state for each domain. While this is often 0,
                           relying on this is not architecture-aware.

The new interface, is designed to abstract this complexity entirely. Also,
applies the hardware's guaranteed minimum supported CBM (based on
r->cache.min_cbm_bits) to each domain automatically.

This unified approach provides a robust and efficient mechanism to
guarantee a full, architecture-aware reset across all domains, eliminating
the need for userspace to parse the topology and construct a complex,
domain-specific input string.

Please let me know if this clarifies the motivation.

Kind regards,
-- 
Aaron Tomlin