linux-kernel - Re: [RFC PATCH 7/8] x86/lib_user_wait.h: Add APIs for user wait instructions

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Message-ID: <alpine.DEB.2.21.1806200032430.10546@nanos.tec.linutronix.de>
Date:   Wed, 20 Jun 2018 00:34:32 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Fenghua Yu <fenghua.yu@...el.com>
cc:     Ingo Molnar <mingo@...hat.com>, H Peter Anvin <hpa@...or.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Alan Cox <alan@...ux.intel.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [RFC PATCH 7/8] x86/lib_user_wait.h: Add APIs for user wait
 instructions

On Tue, 19 Jun 2018, Fenghua Yu wrote:
> On Tue, Jun 19, 2018 at 11:12:05AM +0200, Thomas Gleixner wrote:
> > On Fri, 15 Jun 2018, Fenghua Yu wrote:
> > 
> > > A few new user wait instructions UMONITOR, UMWAIT, and TPAUSE are
> > > published in the latest Intel Instruction Set Extensions document.
> > > 
> > > Define the APIs for user or kernel to use the instructions.
> > 
> > You're not defining APIs. You're adding a pile of misdesigned helper
> > functions which again add static storage per compilation unit and CPUID
> > fiddling.
> > 
> > If you want to add proper APIs then add the stuff to the VDSO and be done
> > with it.
> 
> The user wait instructions are mainly called by user apps; but they can
> be used in kernel as well.
> 
> I'm planning to provide five APIs to user:
> 1. If user wait feature is supported
> 2. nsec to tsc translation
> 3. umonitor function that exectes UMONITOR instruction
> 4. umwait function that executes UMWAIT instruction
> 5. tpause function that executes TPAUSE instruction
> 
> Seems 1-2 can be implemented in VDSO. But should I implement 3-5 in
> VDSO/kernel as well?

If you want a real API, then yes. If not, then an UAPI header is definitely
not the place for this.

Thanks

	tglx