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Date: Wed, 11 Jul 2012 12:17:49 +0200
From: Christian Borntraeger <borntraeger@de.ibm.com>
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To: Avi Kivity <avi@redhat.com>
CC: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.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
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On 11/07/12 11:06, Avi Kivity 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
- if lock holder is not running and we looped for a while do a directed
  yield to that cpu.

> 
>> So there is no win here, but there are other cases were diag44 is used, e.g. cpu_relax.
>> I have to double check with others, if these cases are critical, but for now, it seems 
>> that your dummy implementation  for s390 is just fine. After all it is a no-op until 
>> we implement something.
> 
> Does the data structure make sense for you?  If so we can move it to
> common code (and manage it in kvm_vcpu_on_spin()).  We can guard it with
> CONFIG_KVM_HAVE_CPU_RELAX_INTERCEPT or something, so other archs don't
> have to pay anything.

Ignoring the name, yes the data structure itself seems based on the algorithm
and not on arch specific things. That should work. If we move that to common 
code then s390 will use that scheme automatically for the cases were we call 
kvm_vcpu_on_spin(). All others archs as well.

So this would probably improve guests that uses cpu_relax, for example
stop_machine_run. I have no measurements, though.

Christian