linux-kernel - Re: [RFC PATCH] arm64: Implement cpu

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Message-ID: <20150225132437.GD12377@arm.com>
Date:	Wed, 25 Feb 2015 13:24:38 +0000
From:	Will Deacon <will.deacon@....com>
To:	Peter Crosthwaite <peter.crosthwaite@...inx.com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"michals@...inx.com" <michals@...inx.com>
Subject: Re: [RFC PATCH] arm64: Implement cpu_relax as yield

On Tue, Feb 24, 2015 at 11:07:37PM +0000, Peter Crosthwaite wrote:
> ARM64 has the yield nop hint which has the intended semantics of
> cpu_relax. Implement.
> 
> The immediate application is ARM CPU emulators. An emulator can take
> advantage of the yield hint to de-prioritise an emulated CPU in favor
> of other emulation tasks. QEMU A64 SMP emulation has yield awareness,
> and sees a significant boot time performance increase with this change.

Could you elaborate on the QEMU SMP boot case please? Usually SMP pens
for booting make use of wfe/sev to minimise the spinning overhead.

Will
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