Re: [Xen-devel] [PATCH RFC V11 15/18] kvm : Paravirtual ticketlocks support for linux guests running on KVM hypervisor

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

On 07/23/2013 08:37 PM, Gleb Natapov wrote:
> On Mon, Jul 22, 2013 at 11:50:16AM +0530, Raghavendra K T wrote:
> > +static void kvm_lock_spinning(struct arch_spinlock *lock, __ticket_t want)
[...]
> > +
> > +       /*
> > +        * halt until it's our turn and kicked. Note that we do safe halt
> > +        * for irq enabled case to avoid hang when lock info is overwritten
> > +        * in irq spinlock slowpath and no spurious interrupt occur to save us.
> > +        */
> > +       if (arch_irqs_disabled_flags(flags))
> > +               halt();
> > +       else
> > +               safe_halt();
> > +
> > +out:
> So here now interrupts can be either disabled or enabled. Previous
> version disabled interrupts here, so are we sure it is safe to have them
> enabled at this point? I do not see any problem yet, will keep thinking.
If we enable interrupt here, then


> > +       cpumask_clear_cpu(cpu, &waiting_cpus);
and if we start serving lock for an interrupt that came here,
cpumask clear and w->lock=null may not happen atomically.
if irq spinlock does not take slow path we would have non null value for 
lock, but with no information in waitingcpu.
I am still thinking what would be problem with that.

> > +       w->lock = NULL;
> > +       local_irq_restore(flags);
> > +       spin_time_accum_blocked(start);
> > +}
> > +PV_CALLEE_SAVE_REGS_THUNK(kvm_lock_spinning);
> > +
> > +/* Kick vcpu waiting on @lock->head to reach value @ticket */
> > +static void kvm_unlock_kick(struct arch_spinlock *lock, __ticket_t ticket)
> > +{
> > +       int cpu;
> > +
> > +       add_stats(RELEASED_SLOW, 1);
> > +       for_each_cpu(cpu, &waiting_cpus) {
> > +               const struct kvm_lock_waiting *w = &per_cpu(lock_waiting, cpu);
> > +               if (ACCESS_ONCE(w->lock) == lock &&
> > +                   ACCESS_ONCE(w->want) == ticket) {
> > +                       add_stats(RELEASED_SLOW_KICKED, 1);
> > +                       kvm_kick_cpu(cpu);
> What about using NMI to wake sleepers? I think it was discussed, but
> forgot why it was dismissed.
I think I have missed that discussion. 'll go back and check. so what is 
the idea here? we can easily wake up the halted vcpus that have 
interrupt disabled?

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