linux-kernel - Re: [PATCH 07/18] x86/resctrl: Move CLOSID/RMID matching and setting to use helpers

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Message-ID: <xhsmh8rk8xnxg.mognet@vschneid.remote.csb>
Date:   Fri, 18 Nov 2022 15:49:47 +0000
From:   Valentin Schneider <vschneid@...hat.com>
To:     James Morse <james.morse@....com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Cc:     Fenghua Yu <fenghua.yu@...el.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Babu Moger <Babu.Moger@....com>,
        James Morse <james.morse@....com>,
        shameerali.kolothum.thodi@...wei.com,
        D Scott Phillips OS <scott@...amperecomputing.com>,
        carl@...amperecomputing.com, lcherian@...vell.com,
        bobo.shaobowang@...wei.com, tan.shaopeng@...itsu.com,
        Jamie Iles <quic_jiles@...cinc.com>,
        Xin Hao <xhao@...ux.alibaba.com>, xingxin.hx@...nanolis.org,
        baolin.wang@...ux.alibaba.com, peternewman@...gle.com
Subject: Re: [PATCH 07/18] x86/resctrl: Move CLOSID/RMID matching and
 setting to use helpers

On 21/10/22 13:11, James Morse wrote:
> When switching tasks, the CLOSID and RMID that the new task should
> use are stored in struct task_struct. For x86 the CLOSID known by resctrl,
> the value in task_struct, and the value written to the CPU register are
> all the same thing.
>
> MPAM's CPU interface has two different PARTID's one for data accesses
> the other for instruction fetch. Storing resctrl's CLOSID value in
> struct task_struct implies the arch code knows whether resctrl is using
> CDP.
>
> Move the matching and setting of the struct task_struct properties
> to use helpers. This allows arm64 to store the hardware format of
> the register, instead of having to convert it each time.
>
> __rdtgroup_move_task()s use of READ_ONCE()/WRITE_ONCE() ensures torn
> values aren't seen as another CPU may schedule the task being moved
> while the value is being changed. MPAM has an additional corner-case
> here as the PMG bits extend the PARTID space. If the scheduler sees a
> new-CLOSID but old-RMID, the task will dirty an RMID that the limbo code
> is not watching causing an inaccurate count. x86's RMID are independent
> values, so the limbo code will still be watching the old-RMID in this
> circumstance.
> To avoid this, arm64 needs both the CLOSID/RMID WRITE_ONCE()d together.
> Both values must be provided together.
>

I think I remember something about this... Is that about having them
union'd and read/written as one? (just for my own curiosity)

> Because MPAM's RMID values are not unique, the CLOSID must be provided
> when matching the RMID.
>
> CC: Valentin Schneider <vschneid@...hat.com>
> Signed-off-by: James Morse <james.morse@....com>

Reviewed-by: Valentin Schneider <vschneid@...hat.com>