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

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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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