linux-kernel - Re: [PATCHv10 09/15] x86: Expose untagging mask in /proc/$PID/arch

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Date:   Tue, 18 Oct 2022 15:41:43 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        Kostya Serebryany <kcc@...gle.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Taras Madan <tarasmadan@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "H . J . Lu" <hjl.tools@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Bharata B Rao <bharata@....com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Ashok Raj <ashok.raj@...el.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv10 09/15] x86: Expose untagging mask in
 /proc/$PID/arch_status

On 10/18/22 15:24, Kirill A. Shutemov wrote:
> On Tue, Oct 18, 2022 at 02:02:43PM -0700, Dave Hansen wrote:
>> On 10/18/22 04:33, Kirill A. Shutemov wrote:
>>> Add a line in /proc/$PID/arch_status to report untag_mask. It can be
>>> used to find out LAM status of the process from the outside. It is
>>> useful for debuggers.
>> Considering that address masking is not x86-specific, it seems like this
>> needs a better home (another file in /proc).
> In generic /proc/$PID/status?

Seems like a sane place to me.

> And I'm not sure if it is a good idea at this stage. Semantics around tags
> is not settled across architectures: somewhere it is per-thread, somewhere
> per-process, somewhere it is global.
> 
> Maybe keep it arch-specific?

Yeah, but all of those things could be served by a thread-specific ABI.
The per-thread ABI won't enumerate the scope of the thing, of course.
But, it _can_ precisely communicate what semantics the thread has.