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

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Message-ID: <1866208529.41181.1507988128067.JavaMail.zimbra@efficios.com>
Date:   Sat, 14 Oct 2017 13:35:28 +0000 (UTC)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Andi Kleen <andi@...stfloor.org>
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 <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Chris Lameter <cl@...ux.com>, Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ben Maurer <bmaurer@...com>,
        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 <linux-api@...r.kernel.org>
Subject: Re: [RFC PATCH for 4.15 09/14] Provide cpu_opv system call

----- On Oct 13, 2017, at 10:50 PM, Andi Kleen andi@...stfloor.org wrote:

>> +	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.

Good point!

I'll add this in the next round. Meanwhile, here is the resulting
commit: https://github.com/compudj/linux-percpu-dev/commit/864b444f64f4c227ddc587d12631ff0d3440796c

Thanks,

Mathieu


> 
> -Andi

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com