linux-kernel - Re: [PATCH 2/3] KVM: use separate generations for each address space

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Message-ID: <2103533406.23066784.1487320097082.JavaMail.zimbra@redhat.com>
Date:   Fri, 17 Feb 2017 03:28:17 -0500 (EST)
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Bandan Das <bsd@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH 2/3] KVM: use separate generations for each address
 space

> > +		/*
> > +		 * Generations must be different for each address space.
> > +		 * Init kvm generation close to the maximum to easily test the
> > +		 * code of handling generation number wrap-around.
> > +		 */
> > +		slots->generation = i * 2 - 150;
> > +		rcu_assign_pointer(kvm->memslots[i], slots);
> >  	}
> 
> I can't seem to understand why rcu_assign_pointer wasn't used before.
> kvm->memslots[i] was a rcu protected pointer even before this change,
> right ?

Actually, a better match is RCU_INIT_POINTER.  Here there is no concurrent
reader because we're just initializing the struct kvm.  There is something
else providing synchronization between this writer and the "first" RCU
read-side.  It could be signaling a condition variable, creating a thread,
or releasing a mutex; all three of them have release semantics, which
means they imply a smp_wmb just like rcu_assign_pointer does.

Paolo


> >  	if (init_srcu_struct(&kvm->srcu))
> > @@ -870,8 +872,14 @@ static struct kvm_memslots
> > *install_new_memslots(struct kvm *kvm,
> >  	 * Increment the new memslot generation a second time. This prevents
> >  	 * vm exits that race with memslot updates from caching a memslot
> >  	 * generation that will (potentially) be valid forever.
> > +	 *
> > +	 * Generations must be unique even across address spaces.  We do not need
> > +	 * a global counter for that, instead the generation space is evenly
> > split
> > +	 * across address spaces.  For example, with two address spaces, address
> > +	 * space 0 will use generations 0, 4, 8, ... while * address space 1 will
> > +	 * use generations 2, 6, 10, 14, ...
> >  	 */
> > -	slots->generation++;
> > +	slots->generation += KVM_ADDRESS_SPACE_NUM * 2 - 1;
> >  
> >  	kvm_arch_memslots_updated(kvm, slots);
>