linux-kernel - Re: [PATCH v2 22/45] arm_mpam: resctrl: Add CDP emulation

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Message-ID: <22eadfd1-67f9-470f-8e4e-b9af4dc869e1@arm.com>
Date: Tue, 6 Jan 2026 11:21:20 +0000
From: Ben Horgan <ben.horgan@....com>
To: Jonathan Cameron <jonathan.cameron@...wei.com>
Cc: amitsinght@...vell.com, baisheng.gao@...soc.com,
 baolin.wang@...ux.alibaba.com, carl@...amperecomputing.com,
 dave.martin@....com, david@...nel.org, dfustini@...libre.com,
 fenghuay@...dia.com, gshan@...hat.com, james.morse@....com,
 kobak@...dia.com, lcherian@...vell.com,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 peternewman@...gle.com, punit.agrawal@....qualcomm.com,
 quic_jiles@...cinc.com, reinette.chatre@...el.com, rohit.mathew@....com,
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 joey.gouly@....com, suzuki.poulose@....com, kvmarm@...ts.linux.dev
Subject: Re: [PATCH v2 22/45] arm_mpam: resctrl: Add CDP emulation

Hi Jonathan,

On 1/5/26 18:07, Jonathan Cameron wrote:
> On Fri, 19 Dec 2025 18:11:24 +0000
> Ben Horgan <ben.horgan@....com> wrote:
> 
>> From: James Morse <james.morse@....com>
>>
>> Intel RDT's CDP feature allows the cache to use a different control value
>> depending on whether the accesses was for instruction fetch or a data
>> access. MPAM's equivalent feature is the other way up: the CPU assigns a
>> different partid label to traffic depending on whether it was instruction
>> fetch or a data access, which causes the cache to use a different control
>> value based solely on the partid.
>>
>> MPAM can emulate CDP, with the side effect that the alternative partid is
>> seen by all MSC, it can't be enabled per-MSC.
>>
>> Add the resctrl hooks to turn this on or off. Add the helpers that match a
>> closid against a task, which need to be aware that the value written to
>> hardware is not the same as the one resctrl is using.
>>
>> Update the 'arm64_mpam_global_default' variable the arch code uses during
>> context switch to know when the per-cpu value should be used instead.
>>
>> Awkwardly, the MB controls don't implement CDP. To emulate this, the MPAM
>> equivalent needs programming twice by the resctrl glue, as resctrl expects
>> the bandwidth controls to be applied independently for both data and
>> instruction-fetch.
>>
>> CC: Dave Martin <Dave.Martin@....com>
>> CC: Amit Singh Tomar <amitsinght@...vell.com>
>> Signed-off-by: James Morse <james.morse@....com>
>> Signed-off-by: Ben Horgan <ben.horgan@....com>
> 
> Minor thing below.
> Reviewed-by: Jonathan Cameron <jonathan.cameron@...wei.com>
> 
> 
>> diff --git a/include/linux/arm_mpam.h b/include/linux/arm_mpam.h
>> index 5a78299ec464..ba0312b55d9f 100644
>> --- a/include/linux/arm_mpam.h
>> +++ b/include/linux/arm_mpam.h
>> @@ -5,6 +5,7 @@
>>  #define __LINUX_ARM_MPAM_H
>>  
>>  #include <linux/acpi.h>
>> +#include <linux/resctrl_types.h>
> 
> This seems odd as nothing added to the header in this path appears to need it.
> Maybe should be down in the c file or in a different patch?

Yep, moved it to patch 33, arm_mpam: resctrl: Allow resctrl to allocate
monitors.

> 
>>  #include <linux/types.h>
>>  
>>  struct mpam_msc;
>> @@ -56,6 +57,8 @@ void resctrl_arch_set_cpu_default_closid(int cpu, u32 closid);
>>  void resctrl_arch_set_closid_rmid(struct task_struct *tsk, u32 closid, u32 rmid);
>>  void resctrl_arch_set_cpu_default_closid_rmid(int cpu, u32 closid, u32 rmid);
>>  void resctrl_arch_sched_in(struct task_struct *tsk);
>> +bool resctrl_arch_match_closid(struct task_struct *tsk, u32 closid);
>> +bool resctrl_arch_match_rmid(struct task_struct *tsk, u32 closid, u32 rmid);
>>  
>>  /**
>>   * mpam_register_requestor() - Register a requestor with the MPAM driver
> 

Thanks,

Ben