From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andi Kleen <andi-Vw/NltI1exuRpAAqCnN02g@public.gmane.org>
Subject: Re: [RFC PATCH for 4.15 09/14] Provide cpu_opv system call
Date: Fri, 13 Oct 2017 19:50:30 -0700
Message-ID: <20171014025029.GL2482@two.firstfloor.org>
References: <20171012230326.19984-1-mathieu.desnoyers@efficios.com>
 <20171012230326.19984-10-mathieu.desnoyers@efficios.com>
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To: Mathieu Desnoyers <mathieu.desnoyers-vg+e7yoeK/dWk0Htik3J/w@public.gmane.org>
Cc: "Paul E. McKenney" <paulmck-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>, Boqun Feng <boqun.feng-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>, Peter Zijlstra <peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>, Paul Turner <pjt-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>, Andrew Hunter <ahh-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>, Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org>, Dave Watson <davejwatson-b10kYP2dOMg@public.gmane.org>, Josh Triplett <josh-iaAMLnmF4UmaiuxdJuQwMA@public.gmane.org>, Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Thomas Gleixner <tglx-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>, Andi Kleen <andi-Vw/NltI1exuRpAAqCnN02g@public.gmane.org>, Chris Lameter <cl-vYTEC60ixJUAvxtiuMwx3w@public.gmane.org>, Ingo Molnar <mingo-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>, "H. Peter Anvin" <hpa-YMNOUZJC4hwAvxtiuMwx3w@public.gmane.org>, Ben Maurer <bmaurer-b10kYP2dOMg@public.gmane.org>, Steven Rostedt <rostedt-nx8X9YLhiw1AfugRpC6u6w@public.gmane.org>, Linus Torvalds <torvalds-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, Russell King <linux-lFZ/pmaqli7XmaaqVzeoHQ@public.gmane.org>, Catalin Marinas <catalin.marinas-5wv7dgnIgG8@public.gmane.org>
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> +	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