linux-kernel - Re: [PATCH RFC V10 15/18] kvm : Paravirtual ticketlocks support for linux guests running on KVM hypervisor

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Date:	Wed, 17 Jul 2013 18:25:05 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	mingo@...hat.com, jeremy@...p.org, x86@...nel.org,
	konrad.wilk@...cle.com, hpa@...or.com, pbonzini@...hat.com,
	linux-doc@...r.kernel.org, habanero@...ux.vnet.ibm.com,
	xen-devel@...ts.xensource.com, peterz@...radead.org,
	mtosatti@...hat.com, stefano.stabellini@...citrix.com,
	andi@...stfloor.org, ouyang@...pitt.edu, 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, riel@...hat.com, drjones@...hat.com,
	virtualization@...ts.linux-foundation.org,
	srivatsa.vaddagiri@...il.com
Subject: Re: [PATCH RFC V10 15/18] kvm : Paravirtual ticketlocks support for
 linux guests running on KVM hypervisor

On 07/17/2013 06:15 PM, Gleb Natapov wrote:
> On Wed, Jul 17, 2013 at 03:35:37PM +0530, Raghavendra K T wrote:
>>>> Instead of halt we started with a sleep hypercall in those
>>>>   versions. Changed to halt() once Avi suggested to reuse existing sleep.
>>>>
>>>> If we use older hypercall with few changes like below:
>>>>
>>>> kvm_pv_wait_for_kick_op(flags, vcpu, w->lock )
>>>> {
>>>>   // a0 reserved for flags
>>>> if (!w->lock)
>>>> return;
>>>> DEFINE_WAIT
>>>> ...
>>>> end_wait
>>>> }
>>>>
>>> How would this help if NMI takes lock in critical section. The thing
>>> that may happen is that lock_waiting->want may have NMI lock value, but
>>> lock_waiting->lock will point to non NMI lock. Setting of want and lock
>>> have to be atomic.
>>
>> True. so we are here
>>
>>          non NMI lock(a)
>>          w->lock = NULL;
>>          smp_wmb();
>>          w->want = want;
>>                                 NMI
>>                           <---------------------
>>                            NMI lock(b)
>>                            w->lock = NULL;
>>                            smp_wmb();
>>                            w->want = want;
>>                            smp_wmb();
>>                            w->lock = lock;
>>                           ---------------------->
>>          smp_wmb();
>>          w->lock = lock;
>>
>> so how about fixing like this?
>>
>> again:
>>          w->lock = NULL;
>>          smp_wmb();
>>          w->want = want;
>>          smp_wmb();
>>          w->lock = lock;
>>
>> if (!lock || w->want != want) goto again;
>>
> NMI can happen after the if() but before halt and the same situation
> we are trying to prevent with IRQs will occur.

True, we can not fix that. I thought to fix the inconsistency of
lock,want pair.
But NMI could happen after the first OR condition also.
/me thinks again

  But if NMI handler do not
> take locks we shouldn't worry.

Okay. Thanks for the reviews.
'll spin the next version with all the suggested changes.

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