From: "Jiang Liu (Gerry)" <jiang.liu@huawei.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Jiang Liu <liuj97@gmail.com>, Will Deacon <will.deacon@arm.com>,
"Catalin Marinas" <Catalin.Marinas@arm.com>,
Sandeepa Prabhu <sandeepa.prabhu@linaro.org>,
Marc Zyngier <Marc.Zyngier@arm.com>,
"Arnd Bergmann" <arnd@arndb.de>,
"linux-arm-kernel@lists.infradead.org"
<linux-arm-kernel@lists.infradead.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v3 6/7] arm64, jump label: optimize jump label implementation
Date: Fri, 18 Oct 2013 11:31:22 +0800 [thread overview]
Message-ID: <5260AB8A.2060505@huawei.com> (raw)
In-Reply-To: <20131017112713.2638910f@gandalf.local.home>
On 2013/10/17 23:27, Steven Rostedt wrote:
> On Thu, 17 Oct 2013 22:40:32 +0800
> Jiang Liu <liuj97@gmail.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
next prev parent reply other threads:[~2013-10-18 3:34 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-10-16 3:18 [PATCH v3 0/7] Optimize jump label implementation for ARM64 Jiang Liu
2013-10-16 3:18 ` [PATCH v3 1/7] arm64: introduce basic aarch64 instruction decoding helpers Jiang Liu
2013-10-16 10:51 ` Will Deacon
2013-10-16 15:36 ` Jiang Liu
2013-10-16 17:14 ` Jiang Liu
2013-10-17 9:43 ` Will Deacon
2013-10-16 3:18 ` [PATCH v3 2/7] arm64: introduce interfaces to hotpatch kernel and module code Jiang Liu
2013-10-16 11:11 ` Will Deacon
2013-10-16 16:15 ` Jiang Liu
2013-10-16 3:18 ` [PATCH v3 3/7] arm64: move encode_insn_immediate() from module.c to insn.c Jiang Liu
2013-10-16 11:22 ` Will Deacon
2013-10-16 16:33 ` Jiang Liu
2013-10-16 3:18 ` [PATCH v3 4/7] arm64: introduce aarch64_insn_gen_{nop|branch_imm}() helper functions Jiang Liu
2013-10-16 3:18 ` [PATCH v3 5/7] arm64, jump label: detect %c support for ARM64 Jiang Liu
2013-10-16 3:18 ` [PATCH v3 6/7] arm64, jump label: optimize jump label implementation Jiang Liu
2013-10-16 11:46 ` Will Deacon
2013-10-16 17:11 ` Jiang Liu
2013-10-17 9:39 ` Will Deacon
2013-10-17 14:40 ` Jiang Liu
2013-10-17 15:27 ` Steven Rostedt
2013-10-18 3:31 ` Jiang Liu (Gerry) [this message]
2013-10-18 10:02 ` Will Deacon
2013-10-16 3:18 ` [PATCH v3 7/7] jump_label: use defined macros instead of hard-coding for better readability Jiang Liu
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