From mboxrd@z Thu Jan  1 00:00:00 1970
From: Avi Kivity <avi@redhat.com>
Subject: Re: [PATCH RFC 0/2] kvm: Improving directed yield in PLE handler
Date: Wed, 11 Jul 2012 14:23:09 +0300
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To: Alexander Graf <agraf@suse.de>
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>, 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>, Paul Mackerras <paulus@samba.org>, Benjamin Herrenschmidt <benh@kernel.crashing.org>
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On 07/11/2012 02:16 PM, Alexander Graf wrote:
>> 
>>> 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.
>> 
>> ARM doesn't have an instruction for cpu_relax(), so it can't intercept
>> it.  Given ppc's dislike of overcommit,
> 
> What dislike of overcommit?

I understood ppc virtualization is more of the partitioning sort.
Perhaps I misunderstood it.  But the reliance on device assignment, the
restrictions on scheduling, etc. all point to it.

> 
>> and the way it implements cpu_relax() by adjusting hw thread priority,
> 
> Yeah, I don't think we can intercept relaxing.

... and the lack of ability to intercept cpu_relax() ...

> It's basically a nop-like instruction that gives hardware hints on its current priorities.

That's what x88 PAUSE does.  But we can intercept it (and not just any
execution - we can restrict intercept to tight loops executed more than
a specific number of times).

> That said, we can always add PV code.

Sure, but that's defeated by advancements like self-tuning PLE exits.
It's hard to get this right.

-- 
error compiling committee.c: too many arguments to function