linux-kernel - Re: [PATCH v3 6/7] arm64, jump label: optimize jump label implementation

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Message-ID: <5260AB8A.2060505@huawei.com>
Date:	Fri, 18 Oct 2013 11:31:22 +0800
From:	"Jiang Liu (Gerry)" <jiang.liu@...wei.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Jiang Liu <liuj97@...il.com>, Will Deacon <will.deacon@....com>,
	"Catalin Marinas" <Catalin.Marinas@....com>,
	Sandeepa Prabhu <sandeepa.prabhu@...aro.org>,
	Marc Zyngier <Marc.Zyngier@....com>,
	"Arnd Bergmann" <arnd@...db.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 6/7] arm64, jump label: optimize jump label implementation


On 2013/10/17 23:27, Steven Rostedt wrote:
> On Thu, 17 Oct 2013 22:40:32 +0800
> Jiang Liu <liuj97@...il.com> wrote:
>
>
>>>>> You could make the code more concise by limiting your patching ability to
>>>>> branch immediates. Then a nop is simply a branch to the next instruction (I
>>>>> doubt any modern CPUs will choke on this, whereas the architecture requires
>>>>> a NOP to take time).
>>>> I guess a NOP should be more effecient than a "B #4" on real CPUs:)
>>>
>>> Well, I was actually questioning that. A NOP *has* to take time (the
>>> architecture prevents implementations from discaring it) whereas a static,
>>> unconditional branch will likely be discarded early on by CPUs with even
>>> simple branch prediction logic.
>> I naively thought "NOP" is cheaper than a "B" :(
>> Will use a "B #1" to replace "NOP".
>>
>
> Really?? What's the purpose of a NOP then? It seems to me that an
> architecture is broken if a NOP is slower than a static branch.
>
> -- Steve
Hi Steve and Will,
	I have discussed this with one of our chip design members.
He thinks "NOP" should be better than "B #1" because jump instruction
is one of the most complex instructions for microarchitecture, which
may stall the pipeline. And NOP should be friendly enough for all
microarchitectures. So I will keep the "NOP" version.
Thanks!
Gerry


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