linux-kernel - Re: [PATCH v2 12/29] arm_mpam: Add helpers for managing the locking around the mon

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Message-ID: <2672f409-7923-4b9b-9009-bb5815205e57@arm.com>
Date: Mon, 29 Sep 2025 18:44:54 +0100
From: James Morse <james.morse@....com>
To: Ben Horgan <ben.horgan@....com>, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linux-acpi@...r.kernel.org
Cc: 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>, Dave Martin <dave.martin@....com>,
 Koba Ko <kobak@...dia.com>, Shanker Donthineni <sdonthineni@...dia.com>,
 fenghuay@...dia.com, baisheng.gao@...soc.com,
 Jonathan Cameron <jonathan.cameron@...wei.com>, Rob Herring
 <robh@...nel.org>, Rohit Mathew <rohit.mathew@....com>,
 Rafael Wysocki <rafael@...nel.org>, Len Brown <lenb@...nel.org>,
 Lorenzo Pieralisi <lpieralisi@...nel.org>, Hanjun Guo
 <guohanjun@...wei.com>, Sudeep Holla <sudeep.holla@....com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Danilo Krummrich <dakr@...nel.org>
Subject: Re: [PATCH v2 12/29] arm_mpam: Add helpers for managing the locking
 around the mon_sel registers

Hi Ben,

On 11/09/2025 16:31, Ben Horgan wrote:
> On 9/10/25 21:42, James Morse wrote:
>> The MSC MON_SEL register needs to be accessed from hardirq for the overflow
>> interrupt, and when taking an IPI to access these registers on platforms
>> where MSC are not accesible from every CPU. This makes an irqsave
>> spinlock the obvious lock to protect these registers. On systems with SCMI
>> mailboxes it must be able to sleep, meaning a mutex must be used. The
>> SCMI platforms can't support an overflow interrupt.
>>
>> Clearly these two can't exist for one MSC at the same time.
>>
>> Add helpers for the MON_SEL locking. The outer lock must be taken in a
>> pre-emptible context before the inner lock can be taken. On systems with
>> SCMI mailboxes where the MON_SEL accesses must sleep - the inner lock
>> will fail to be 'taken' if the caller is unable to sleep. This will allow
>> callers to fail without having to explicitly check the interface type of
>> each MSC.

>> diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h
>> index 828ce93c95d5..4cc44d4e21c4 100644
>> --- a/drivers/resctrl/mpam_internal.h
>> +++ b/drivers/resctrl/mpam_internal.h
>> @@ -70,12 +70,17 @@ struct mpam_msc {
>>  
>>  	/*
>>  	 * mon_sel_lock protects access to the MSC hardware registers that are
>> -	 * affected by MPAMCFG_MON_SEL.
>> +	 * affected by MPAMCFG_MON_SEL, and the mbwu_state.
>> +	 * Access to mon_sel is needed from both process and interrupt contexts,
>> +	 * but is complicated by firmware-backed platforms that can't make any
>> +	 * access unless they can sleep.
>> +	 * Always use the mpam_mon_sel_lock() helpers.
>> +	 * Accessed to mon_sel need to be able to fail if they occur in the wrong
>> +	 * context.
>>  	 * If needed, take msc->probe_lock first.
>>  	 */
>> -	struct mutex		outer_mon_sel_lock;
>> -	raw_spinlock_t		inner_mon_sel_lock;
>> -	unsigned long		inner_mon_sel_flags;
>> +	raw_spinlock_t		_mon_sel_lock;
>> +	unsigned long		_mon_sel_flags;
>>  
> 
> These stale variables can be removed in the patch that introduced them,
> outer_mon_sel_lock, inner_mon_sel_lock, inner_mon_sel_flags. Jonathan
> has already pointed out the stale comment and paragraph in the commit
> message.

Yeah - I forgot to rewrite the commit message when I split this patch.
I'll pull those earlier bits into the now:later patch that splits the locking up.


Thanks,

James