linux-kernel - Re: [RFCv2 03/10] x86: Introduce userspace API to handle per-thread features

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Date:   Sun, 15 May 2022 18:24:01 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "hjl.tools@...il.com" <hjl.tools@...il.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "aryabinin@...tuozzo.com" <aryabinin@...tuozzo.com>,
        "dvyukov@...gle.com" <dvyukov@...gle.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "Lutomirski, Andy" <luto@...nel.org>,
        "glider@...gle.com" <glider@...gle.com>
Subject: Re: [RFCv2 03/10] x86: Introduce userspace API to handle per-thread
 features

On Sun, 2022-05-15 at 11:02 +0200, Thomas Gleixner wrote:
> > Otherwise, the option that used to be used here was a "status"
> > arch_prctl(), which was called separately to find out what actually
> > got
> > enabled after an "enable" call. That fit with the GET/SET semantics
> > already in place.
> > 
> > I guess we could also get rid of the batch enabling idea, and just
> > have
> > one "enable" call per feature too. But then it is syscall heavy.
> 
> This is not a runtime hotpath problem. Those prctls() happen once
> when
> the process starts, so having three which are designed for the
> individual purpose instead of one ill defined is definitely the
> better
> choice.
> 
> Premature optimization is never a good idea. Keep it simple is the
> right
> starting point.
> 
> If it really turns out to be something which matters, then you can
> provide a batch interface later on if it makes sense to do so, but
> see
> above.

Thanks, sounds good to me.

Kirill, so I guess we can just change ARCH_THREAD_FEATURE_ENABLE/
ARCH_THREAD_FEATURE_DISABLE to return EINVAL if more than one bit is
set. It returns 0 on success and whatever error code on failure.
Userspace can do whatever rollback logic it wants. What do you think?