linux-kernel - Re: [RFC PATCH 00/19] x86,fs/resctrl: Support for Global Bandwidth Enforcement and Priviledge Level Zero Association

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Message-ID: <aYJTfc5g_qgn--eK@agluck-desk3>
Date: Tue, 3 Feb 2026 11:58:53 -0800
From: "Luck, Tony" <tony.luck@...el.com>
To: Babu Moger <babu.moger@....com>, Drew Fustini <fustini@...nel.org>, "James
 Morse" <james.morse@....com>, Dave Martin <Dave.Martin@....com>
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Subject: Re: [RFC PATCH 00/19] x86,fs/resctrl: Support for Global Bandwidth
 Enforcement and Priviledge Level Zero Association

On Wed, Jan 21, 2026 at 03:12:38PM -0600, Babu Moger wrote:
> Privilege Level Zero Association (PLZA) 
> 
> Privilege Level Zero Association (PLZA) allows the hardware to
> automatically associate execution in Privilege Level Zero (CPL=0) with a
> specific COS (Class of Service) and/or RMID (Resource Monitoring
> Identifier). The QoS feature set already has a mechanism to associate
> execution on each logical processor with an RMID or COS. PLZA allows the
> system to override this per-thread association for a thread that is
> executing with CPL=0. 

Adding Drew, and prodding Dave & James, for this discussion.

At LPC it was stated that both ARM and RISC-V already have support
to run kernel code with different quality of service parameters from
user code.

I'm thinking that Babu's implementation for resctrl may be over
engineered. Specifically the part that allows users to put some
tasks into the PLZA group, while leaving others in a mode where
kernel code runs with same QoS parameters as user code.

That comes at a cost of complexity, and performance in the context
switch code.

But maybe I'm missing some practical case where users want that
behaviour.

-Tony