Re: [RFC] KVM: x86: conditionally acquire/release slots_lock on entry/exit

[Avi Kivity <avi@redhat.com>]

On 09/14/2009 08:03 AM, Avi Kivity wrote:
>> Right it will. But this does not stop the fault path from creating
>> shadow pages with stale sp->gfn (the only way to do that would be mutual
>> exclusion AFAICS).
>
> So we put the kvm_mmu_zap_pages() call as part of the 
> synchronize_srcu() callback to take advantage of the srcu guarantees.  
> We know that when when the callback is called all new reads see the 
> new slots and all old readers have completed.

I think I see your concern - assigning sp->gfn leaks information out of 
the srcu critical section.

Two ways out:

1) copy kvm->slots into sp->slots and use it when dropping the shadow 
page.  Intrusive and increases shadow footprint.

1b) Instead of sp->slots, use a 1-bit generation counter.  Even uglier 
but reduces the shadow footprint.

2) instead of removing the slot in rcu_assign_pointer(), mark it 
invalid.  gfn_to_page() will fail on such slots but the teardown paths 
(like unaccount_shadow) continue to work.  One we've zapped the mmu we 
drop the slot completely (can do in place, no need to rcu_assign_pointer).

-- 
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