From mboxrd@z Thu Jan  1 00:00:00 1970
From: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Subject: Re: [PATCH RFC V4 5/5] Documentation/kvm : Add documentation on
	Hypercalls and features used for PV spinlock
Date: Fri, 20 Jan 2012 20:39:55 +0530
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To: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>, Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>, KVM <kvm@vger.kernel.org>, linux-doc@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>, Jan Kiszka <jan.kiszka@siemens.com>, Virtualization <virtualization@lists.linux-foundation.org>, Anthony Liguori <aliguori@linux.vnet.ibm.com>, Paul Mackerras <paulus@samba.org>, "H. Peter Anvin" <hpa@zytor.com>, Stefano Stabellini <stefano.stabellini@eu.citrix.com>, Xen <xen-devel@lists.xensource.com>, Dave Jiang <dave.jiang@intel.com>, Glauber Costa <glommer@redhat.com>, X86 <x86@kernel.org>, Ingo Molnar <mingo@redhat.com>, Avi Kivity <avi@redhat.com>, Rik van Riel <riel@redhat.com>, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>, Sasha Levin <levinsasha928@gmail.com>, Sedat Dilek <sedat.dilek@gmail.com>, Thomas Gleixner <tglx@linutronix.de>, Anthony Liguori <aliguori@us.ibm.com>, Greg
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* Marcelo Tosatti <mtosatti@redhat.com> [2012-01-17 13:53:03]:

> on tue, jan 17, 2012 at 05:32:33pm +0200, gleb natapov wrote:
> > on tue, jan 17, 2012 at 07:58:18pm +0530, srivatsa vaddagiri wrote:
> > > * gleb natapov <gleb@redhat.com> [2012-01-17 15:20:51]:
> > > 
> > > > > having the hypercall makes the intent of vcpu (to sleep on a kick) clear to 
> > > > > hypervisor vs assuming that because of a trapped hlt instruction (which
> > > > > will anyway won't work when yield_on_hlt=0).
> > > > > 
> > > > the purpose of yield_on_hlt=0 is to allow vcpu to occupy cpu for the
> > > > entire time slice no mater what. i do not think disabling yield on hlt
> > > > is even make sense in cpu oversubscribe scenario.
> > > 
> > > Yes, so is there any real use for yield_on_hlt=0? I believe Anthony
> > > initially added it as a way to implement CPU bandwidth capping for VMs,
> > > which would ensure that busy VMs don't eat into cycles meant for a idle
> > > VM. Now that we have proper support in scheduler for CPU bandwidth capping, is 
> > > there any real world use for yield_on_hlt=0? If not, deprecate it?
> > > 
> > I was against adding it in the first place, so if IBM no longer needs it
> > I am for removing it ASAP.
> 
> +1. 
> 
> Anthony?

CCing Anthony.

Anthony, could you ACK removal of yield_on_hlt (as keeping it around will
require unnecessary complications in pv-spinlock patches)?

- vatsa