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

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Message-ID: <CALPaoCgDuA1D+ubUv3UJ0+LyUfjCFKOcrEhg6Q80rNK5idjEmA@mail.gmail.com>
Date: Tue, 9 Dec 2025 16:53:58 +0100
From: Peter Newman <peternewman@...gle.com>
To: Ben Horgan <ben.horgan@....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 Ben,

On Tue, Dec 9, 2025 at 3:40 PM Ben Horgan <ben.horgan@....com> wrote:
>
> Hi James and all,
>
> As James is otherwise occupied, I am planning to post a follow up
> version of this series once it's had time to be reviewed. I will be
> posting my own review comments; please give them extra scrutiny.
>
> On 12/5/25 21:58, James Morse wrote:
> > This is the missing piece to make MPAM usable resctrl in user-space. This has
> > shed its debugfs code and the read/write 'event configuration' for the monitors
> > to make the series smaller.
> >
> [...]
> >
> > This series is based on arm64/for-next/core, and can be retrieved from:
> > https://git.kernel.org/pub/scm/linux/kernel/git/morse/linux.git mpam/glue/rfc
> >
> > There is no snapshot branch - this is it!
> > I'll push the extras branch once I've gotten a handle on the DT shaped mess in
> > there.
>
> 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