linux-kernel - Re: [PATCH v1 20/31] x86/resctrl: Allow an architecture to disable pseudo lock

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Date: Mon, 8 Apr 2024 20:24:12 -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>, D Scott Phillips OS
	<scott@...amperecomputing.com>, <carl@...amperecomputing.com>,
	<lcherian@...vell.com>, <bobo.shaobowang@...wei.com>,
	<tan.shaopeng@...itsu.com>, <baolin.wang@...ux.alibaba.com>, Jamie Iles
	<quic_jiles@...cinc.com>, Xin Hao <xhao@...ux.alibaba.com>,
	<peternewman@...gle.com>, <dfustini@...libre.com>, <amitsinght@...vell.com>,
	David Hildenbrand <david@...hat.com>, Rex Nie <rex.nie@...uarmicro.com>, Dave
 Martin <dave.martin@....com>
Subject: Re: [PATCH v1 20/31] x86/resctrl: Allow an architecture to disable
 pseudo lock

Hi James,

On 3/21/2024 9:50 AM, James Morse wrote:
> Pseudo-lock relies on knowledge of the micro-architecture to disable
> prefetchers etc.
> 
> On arm64 these controls are typically secure only, meaning linux can't
> access them. Arm's cache-lockdown feature works in a very different
> way. Resctrl's pseudo-lock isn't going to be used on arm64 platforms.
> 
> Add a Kconfig symbol that can be selected by the architecture. This
> enables or disables building of the psuedo_lock.c file, and replaces

pseudo_lock.c

> the functions with stubs. An additional IS_ENABLED() check is needed
> in rdtgroup_mode_write() so that attempting to enable pseudo-lock
> reports an "Unknown or unsupported mode" to user-space.
> 

I am missing something here. It is not obvious to me why the IS_ENABLED()
check is needed. Wouldn't rdtgroup_locksetup_enter()
return -EOPNOTSUPP if CONFIG_RESCTRL_FS_PSEUDO_LOCK is not enabled?

Reinette