linux-kernel - Re: [RFC PATCH 00/38] arm_mpam: Add KVM/arm64 and resctrl glue code

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Message-ID: <95c9947f-574a-486f-953c-fbac6ead0853@arm.com>
Date: Tue, 9 Dec 2025 16:14:12 +0000
From: Ben Horgan <ben.horgan@....com>
To: Peter Newman <peternewman@...gle.com>
Cc: James Morse <james.morse@....com>, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org,
 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>, dfustini@...libre.com,
 amitsinght@...vell.com, David Hildenbrand <david@...nel.org>,
 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>,
 Gavin Shan <gshan@...hat.com>, rohit.mathew@....com,
 reinette.chatre@...el.com, Punit Agrawal <punit.agrawal@....qualcomm.com>
Subject: Re: [RFC PATCH 00/38] arm_mpam: Add KVM/arm64 and resctrl glue code

Hi Peter,

On 12/9/25 15:53, Peter Newman wrote:
> Hi Ben,
> 
> On Tue, Dec 9, 2025 at 3:40 PM Ben Horgan <ben.horgan@....com> wrote:
>>
>> Hi James and all,
[...]
>>
>> One major departure from the previous snapshot branches referenced in
>> the base driver series is that the same MPAM setting are used for
>> kernel-space and user-space. That is, MPAM1_EL1 is set to the same value
>> as MPAM0_EL1 rather than keeping the default value. The advantages of
>> this are that it is closer to the x86 model where the closid is globally
>> applicable, all partids are usable from user-space and user-space can't
>> bypass MPAM controls by doing the work in the kernel. However, this
>> causes some priority inversion where a high priority task waits to take
>> a mutex held by another whose resources are restricted by MPAM.
>> Thanks,
> 
> In our experience, the disadvantages of the x86 model were worse
> because they triggered on hosts unintentionally, while making the
> kernel do work unrestricted on behalf of the user at least requires
> intentional abuse.
> 
> -Peter

Thanks for the quick feedback. Do you have any more data/information on
this that you can share?

Thanks,

Ben