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Message-ID: <172f4615-8943-448c-ac68-152d5bcffa76@intel.com>
Date: Thu, 11 Apr 2024 10:40:03 -0700
User-Agent: Mozilla Thunderbird
Subject: Re: [PATCH v1 20/31] x86/resctrl: Allow an architecture to disable
 pseudo lock
To: Dave Martin <Dave.Martin@arm.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>, Ingo Molnar <mingo@redhat.com>, "Borislav
 Petkov" <bp@alien8.de>, H Peter Anvin <hpa@zytor.com>, Babu Moger
	<Babu.Moger@amd.com>, <shameerali.kolothum.thodi@huawei.com>, "D Scott
 Phillips OS" <scott@os.amperecomputing.com>, <carl@os.amperecomputing.com>,
	<lcherian@marvell.com>, <bobo.shaobowang@huawei.com>,
	<tan.shaopeng@fujitsu.com>, <baolin.wang@linux.alibaba.com>, Jamie Iles
	<quic_jiles@quicinc.com>, Xin Hao <xhao@linux.alibaba.com>,
	<peternewman@google.com>, <dfustini@baylibre.com>, <amitsinght@marvell.com>,
	David Hildenbrand <david@redhat.com>, Rex Nie <rex.nie@jaguarmicro.com>
References: <20240321165106.31602-1-james.morse@arm.com>
 <20240321165106.31602-21-james.morse@arm.com>
 <6d563361-87c0-474d-812c-b33e79eacf3a@intel.com>
 <Zhfw/uEZaYk9492j@e133380.arm.com>
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From: Reinette Chatre <reinette.chatre@intel.com>
In-Reply-To: <Zhfw/uEZaYk9492j@e133380.arm.com>
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Hi Dave,

On 4/11/2024 7:17 AM, Dave Martin wrote:
> 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?

Thank you for digging into this. It was not obvious to me that
the changelog referred to the last_cmd_status string. I do
not think this warrants making the stubs more complicated.

Reinette