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Date: Thu, 11 Apr 2024 15:17:34 +0100
From: Dave Martin <Dave.Martin@arm.com>
To: Reinette Chatre <reinette.chatre@intel.com>
Cc: James Morse <james.morse@arm.com>, x86@kernel.org,
	linux-kernel@vger.kernel.org, Fenghua Yu <fenghua.yu@intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
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	H Peter Anvin <hpa@zytor.com>, Babu Moger <Babu.Moger@amd.com>,
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	Xin Hao <xhao@linux.alibaba.com>, peternewman@google.com,
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	David Hildenbrand <david@redhat.com>,
	Rex Nie <rex.nie@jaguarmicro.com>
Subject: Re: [PATCH v1 20/31] x86/resctrl: Allow an architecture to disable
 pseudo lock
Message-ID: <Zhfw/uEZaYk9492j@e133380.arm.com>
References: <20240321165106.31602-1-james.morse@arm.com>
 <20240321165106.31602-21-james.morse@arm.com>
 <6d563361-87c0-474d-812c-b33e79eacf3a@intel.com>
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In-Reply-To: <6d563361-87c0-474d-812c-b33e79eacf3a@intel.com>

On Mon, Apr 08, 2024 at 08:24:12PM -0700, Reinette Chatre wrote:
> 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

Noted.

> > 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
> 

Hmm, if I've understood all this correctly, then it looks like the
existing code in rdtgroup_mode_write() relies on the dispatched
function (rdtgroup_locksetup_enter() etc.) to do an appropriate
rdt_last_cmd_puts() on failure.  If no function is called at all and
the requested mode change is not a no-op or otherwise trivially
successful, then it looks like we're supposed to fall into the else
clause.

I'd guess James' intent here was to use the fallback else {} to write
a suitable status string, while keeping the stub functions as trivial
as possible.

Just taking the IS_ENABLED() away would result in error return from the
write(), but no suitable last_cmd_status string.

For consistency with the existing x86 implementation, I wonder whether
we should put a suitable rdt_last_cmd_puts() in the stub for
rdtgroup_locksetup_enter().

There might be other ways to refactor or simplify this, though.

Thoughts?

Cheers
---Dave