From mboxrd@z Thu Jan  1 00:00:00 1970
From: Gleb Natapov <gleb@redhat.com>
Subject: Re: [PATCH RFC V4 5/5] Documentation/kvm : Add documentation on
	Hypercalls and features used for PV spinlock
Date: Tue, 17 Jan 2012 17:32:33 +0200
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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>,
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	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>, Greg Kroah-Hartman <gregkh@suse.de>,
	LKML <linux-kernel@vger.kernel.org>, Dave Hansen <dave
To: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
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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.

> > Now if you'll call
> > KVM_HC_WAIT_FOR_KICK instead of HLT you will effectively ignore
> > yield_on_hlt=0 setting.
> 
> I guess that depends on what we do in KVM_HC_WAIT_FOR_KICK. If we do
> yield_to() rather than sleep, it should minimize how much cycles vcpu gives away
> to a competing VM (which seems to be the biggest purpose why one may
> want to set yield_on_hlt=0).
> 
> > This is like having PV HLT that does not obey
> > VMX exit control setting.
> 
> - vatsa

--
			Gleb.

From mboxrd@z Thu Jan  1 00:00:00 1970
From: Gleb Natapov <gleb@redhat.com>
Subject: Re: [PATCH RFC V4 5/5] Documentation/kvm : Add documentation on
	Hypercalls and features used for PV spinlock
Date: Tue, 17 Jan 2012 17:32:33 +0200
Message-ID: <20120117153233.GA7911@redhat.com>
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	<20120117091413.GM2167@redhat.com>
	<20120117122650.GC30757@linux.vnet.ibm.com>
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	<20120117131103.GD30398@linux.vnet.ibm.com>
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To: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.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>, 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>, Greg Kroah-Hartman <gregkh@suse.de>, LKML <linux-kernel@vger.kernel.org>Dave Hansen <dave>
List-Id: virtualization@lists.linuxfoundation.org

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.

> > Now if you'll call
> > KVM_HC_WAIT_FOR_KICK instead of HLT you will effectively ignore
> > yield_on_hlt=0 setting.
> 
> I guess that depends on what we do in KVM_HC_WAIT_FOR_KICK. If we do
> yield_to() rather than sleep, it should minimize how much cycles vcpu gives away
> to a competing VM (which seems to be the biggest purpose why one may
> want to set yield_on_hlt=0).
> 
> > This is like having PV HLT that does not obey
> > VMX exit control setting.
> 
> - vatsa

--
			Gleb.