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Date: Wed, 11 Jul 2012 17:34:47 +0530
From: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
Organization: IBM
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To: Christian Borntraeger <borntraeger@de.ibm.com>
CC: Avi Kivity <avi@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Marcelo Tosatti <mtosatti@redhat.com>, Ingo Molnar <mingo@redhat.com>,
        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>
Subject: Re: [PATCH RFC 0/2] kvm: Improving directed yield in PLE handler
References: <20120709062012.24030.37154.sendpatchset@codeblue> <4FFA8E5E.3070108@de.ibm.com> <4FFD422B.9060008@redhat.com> <4FFD52CD.7040403@de.ibm.com> <4FFD68AA.1000607@linux.vnet.ibm.com> <4FFD69B5.9000401@de.ibm.com>
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On 07/11/2012 05:25 PM, Christian Borntraeger wrote:
> On 11/07/12 13:51, Raghavendra K T wrote:
>>>>> Almost all s390 kernels use diag9c (directed yield to a given guest cpu) for spinlocks, though.
>>>>
>>>> Perhaps x86 should copy this.
>>>
>>> See arch/s390/lib/spinlock.c
>>> The basic idea is using several heuristics:
>>> - loop for a given amount of loops
>>> - check if the lock holder is currently scheduled by the hypervisor
>>>     (smp_vcpu_scheduled, which uses the sigp sense running instruction)
>>>     Dont know if such thing is available for x86. It must be a lot cheaper
>>>     than a guest exit to be useful
>>
>> Unfortunately we do not have information on lock-holder.
>
> That would be an independent patch and requires guest changes.
>

Yes, AFAI think, there are two options:
(1) extend lock and use spare bit in ticketlock indicate lock is held
(2) use percpu list entry.