linux-kernel - Re: [PATCH RFC 0/2] kvm: Improving directed yield in PLE handler

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Message-ID: <1342059459.18850.46.camel@pasglop>
Date:	Thu, 12 Jul 2012 12:17:39 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Avi Kivity <avi@...hat.com>
Cc:	Christian Borntraeger <borntraeger@...ibm.com>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Ingo Molnar <mingo@...hat.com>, Rik van Riel <riel@...hat.com>,
	S390 <linux-s390@...r.kernel.org>,
	Carsten Otte <cotte@...ibm.com>, KVM <kvm@...r.kernel.org>,
	chegu vinod <chegu_vinod@...com>,
	"Andrew M. Theurer" <habanero@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>, X86 <x86@...nel.org>,
	Gleb Natapov <gleb@...hat.com>, linux390@...ibm.com,
	Srivatsa Vaddagiri <srivatsa.vaddagiri@...il.com>,
	Joerg Roedel <joerg.roedel@....com>,
	Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com>,
	Alexander Graf <agraf@...e.de>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH RFC 0/2] kvm: Improving directed yield in PLE handler

> ARM doesn't have an instruction for cpu_relax(), so it can't intercept
> it.  Given ppc's dislike of overcommit, and the way it implements
> cpu_relax() by adjusting hw thread priority, I'm guessing it doesn't
> intercept those either, but I'm copying the ppc people in case I'm
> wrong.  So it's s390 and x86.

No but our spinlocks call __spin_yield()  (or __rw_yield) which does
some paravirt tricks already.

We check if the holder is currently running, and if not, we call the
H_CONFER hypercall which can be used to "give" our time slice to the
holder.

Our implementation of H_CONFER in KVM is currently a nop though.

Cheers,
Ben.


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