linux-kernel - Re: [PATCH RFC V9 0/19] Paravirtualized ticket spinlocks

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Date:	Wed, 10 Jul 2013 16:58:29 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Andrew Jones <drjones@...hat.com>, mingo@...hat.com,
	ouyang@...pitt.edu, habanero@...ux.vnet.ibm.com, jeremy@...p.org,
	x86@...nel.org, konrad.wilk@...cle.com, hpa@...or.com,
	pbonzini@...hat.com, linux-doc@...r.kernel.org,
	xen-devel@...ts.xensource.com, mtosatti@...hat.com,
	stefano.stabellini@...citrix.com, andi@...stfloor.org,
	attilio.rao@...rix.com, gregkh@...e.de, agraf@...e.de,
	chegu_vinod@...com, torvalds@...ux-foundation.org,
	avi.kivity@...il.com, tglx@...utronix.de, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, stephan.diestelhorst@....com,
	riel@...hat.com, virtualization@...ts.linux-foundation.org,
	srivatsa.vaddagiri@...il.com
Subject: Re: [PATCH RFC V9 0/19] Paravirtualized ticket spinlocks

On 07/10/2013 04:17 PM, Gleb Natapov wrote:
> On Wed, Jul 10, 2013 at 12:40:47PM +0200, Peter Zijlstra wrote:
>> On Wed, Jul 10, 2013 at 01:33:25PM +0300, Gleb Natapov wrote:
>>
>> Here's an idea, trim the damn email ;-) -- not only directed at gleb.
>>
> Good idea.
>
>>>> Ingo, Gleb,
>>>>
>>>>  From the results perspective, Andrew Theurer, Vinod's test results are
>>>> pro-pvspinlock.
>>>> Could you please help me to know what will make it a mergeable
>>>> candidate?.
>>>>
>>> I need to spend more time reviewing it :) The problem with PV interfaces
>>> is that they are easy to add but hard to get rid of if better solution
>>> (HW or otherwise) appears.
>>
>> How so? Just make sure the registration for the PV interface is optional; that
>> is, allow it to fail. A guest that fails the PV setup will either have to try
>> another PV interface or fall back to 'native'.
>>
> We have to carry PV around for live migration purposes. PV interface
> cannot disappear under a running guest.
>

IIRC, The only requirement was running state of the vcpu to be retained.
This was addressed by
[PATCH RFC V10 13/18] kvm : Fold pv_unhalt flag into GET_MP_STATE ioctl 
to aid migration.

I would have to know more if I missed something here.

>>>> I agree that Jiannan's Preemptable Lock idea is promising and we could
>>>> evaluate that  approach, and make the best one get into kernel and also
>>>> will carry on discussion with Jiannan to improve that patch.
>>> That would be great. The work is stalled from what I can tell.
>>
>> I absolutely hated that stuff because it wrecked the native code.
> Yes, the idea was to hide it from native code behind PV hooks.
>

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