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

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Date:   Sat, 14 May 2022 10:37:25 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "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 Fri, May 13 2022 at 23:50, Edgecombe, Rick P wrote:
> On Sat, 2022-05-14 at 02:09 +0300, Kirill A. Shutemov wrote:
>> On Fri, May 13, 2022 at 05:34:12PM +0000, Edgecombe, Rick P wrote:
>> > On Fri, 2022-05-13 at 16:09 +0200, Alexander Potapenko wrote:
>> > > > +
>> > > > +       /* Handle ARCH_THREAD_FEATURE_ENABLE */
>> > > > +
>> > > > +       task->thread.features |= features;
>> > > > +out:
>> > > > +       return task->thread.features;
>> > > 
>> > > Isn't arch_prctl() supposed to return 0 on success?
>> > 
>> > Hmm, good point. Maybe we'll need a struct to pass info in and out.
>> 
>> But values >0 are unused. I don't see why can't we use them.
>
> Hmm, I don't know what it would break since it is a new "code"
> argument. But the man page says:
> "On success, arch_prctl() returns 0; on error, -1 is returned, and
> errno is set to indicate the error."
>
> So just change the man pages?
> "On success, arch_prctl() returns positive values; on error, -1 is
> returned, and errno is set to indicate the error."

Why?

        prctl(GET, &out)

is the pattern used all over the place.

Thanks,

        tglx