linux-kernel - Re: [RFC PATCH for 4.15 09/14] Provide cpu

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Message-ID: <20171014025029.GL2482@two.firstfloor.org>
Date:   Fri, 13 Oct 2017 19:50:30 -0700
From:   Andi Kleen <andi@...stfloor.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Paul Turner <pjt@...gle.com>, Andrew Hunter <ahh@...gle.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>,
        Josh Triplett <josh@...htriplett.org>,
        Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Andi Kleen <andi@...stfloor.org>, Chris Lameter <cl@...ux.com>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ben Maurer <bmaurer@...com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        linux-api@...r.kernel.org
Subject: Re: [RFC PATCH for 4.15 09/14] Provide cpu_opv system call

> +	pagefault_disable();
> +	switch (len) {
> +	case 1:
> +		if (__get_user(tmp._u8, (uint8_t __user *)p))
> +			goto end;
> +		tmp._u8 += (uint8_t)count;
> +		if (__put_user(tmp._u8, (uint8_t __user *)p))
> +			goto end;
> +		break;

It seems the code size could be dramatically shrunk by using a function
pointer callback for the actual operation, and then avoiding all the
copies. Since this is only a fallback this shouldn't be a problem
(and modern branch predictors are fairly good in such situations anyways)

If you really want to keep the code it would be better if you used
some macros -- i bet there's a typo in here somewhere in this
forest.

-Andi