linux-kernel - Re: [PATCH v2 15/29] arm_mpam: Reset MSC controls from cpu hp callbacks

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Message-ID: <137ca3de-2572-4edf-b10c-67b690f84128@arm.com>
Date: Tue, 30 Sep 2025 18:06:29 +0100
From: James Morse <james.morse@....com>
To: Jonathan Cameron <jonathan.cameron@...wei.com>
Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linux-acpi@...r.kernel.org,
 D Scott Phillips OS <scott@...amperecomputing.com>,
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 baolin.wang@...ux.alibaba.com, Jamie Iles <quic_jiles@...cinc.com>,
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 dfustini@...libre.com, amitsinght@...vell.com,
 David Hildenbrand <david@...hat.com>, Dave Martin <dave.martin@....com>,
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 fenghuay@...dia.com, baisheng.gao@...soc.com, Rob Herring <robh@...nel.org>,
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 Danilo Krummrich <dakr@...nel.org>
Subject: Re: [PATCH v2 15/29] arm_mpam: Reset MSC controls from cpu hp
 callbacks

Hi Jonathan,

On 12/09/2025 12:55, Jonathan Cameron wrote:
> On Wed, 10 Sep 2025 20:42:55 +0000
> James Morse <james.morse@....com> wrote:
> 
>> When a CPU comes online, it may bring a newly accessible MSC with
>> it. Only the default partid has its value reset by hardware, and
>> even then the MSC might not have been reset since its config was
>> previously dirtyied. e.g. Kexec.
>>
>> Any in-use partid must have its configuration restored, or reset.
>> In-use partids may be held in caches and evicted later.
>>
>> MSC are also reset when CPUs are taken offline to cover cases where
>> firmware doesn't reset the MSC over reboot using UEFI, or kexec
>> where there is no firmware involvement.
>>
>> If the configuration for a RIS has not been touched since it was
>> brought online, it does not need resetting again.
>>
>> To reset, write the maximum values for all discovered controls.

> Just one trivial passing comment from me.

>> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
>> index cd8e95fa5fd6..0353313cf284 100644
>> --- a/drivers/resctrl/mpam_devices.c
>> +++ b/drivers/resctrl/mpam_devices.c
>> @@ -818,6 +921,20 @@ static int mpam_discovery_cpu_online(unsigned int cpu)
>>  
>>  static int mpam_cpu_offline(unsigned int cpu)
>>  {
>> +	int idx;
>> +	struct mpam_msc *msc;
>> +
>> +	idx = srcu_read_lock(&mpam_srcu);

> Might be worth using
> guard(srcu)(&mpam_srcu);
> here but only real advantage it bring is in hiding the local idx variable
> away.  

Sure. I did that in a few other places, it looks like I either missed this, or thought it
got more complicated later...


Thanks,

James