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

[Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>]

On 07/17/2013 07:43 PM, Raghavendra K T wrote:
> On 07/17/2013 06:55 PM, Gleb Natapov wrote:
> > On Wed, Jul 17, 2013 at 06:25:05PM +0530, Raghavendra K T wrote:
> > > 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
> > lock_spinning() can check that it is called in nmi context and bail out.
> Good point.
> I think we can check for even irq context and bailout so that in irq
> context we continue spinning instead of slowpath. no ?
>
> > How often this will happens anyway.
> I know NMIs occur frequently with watchdogs. or used by sysrq-trigger
> etc.. But I am not an expert how frequent it is otherwise.
Forgot to ask if Peter has any points on NMI frequency.

But even
> then if they do not use spinlock, we have no problem as already pointed.
>
> I can measure with debugfs counter how often it happens.

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