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