On 04/05/21 19:26, Ben Gardon wrote:
On Mon, May 3, 2021 at 6:42 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
On 29/04/21 23:18, Ben Gardon wrote:
+void activate_shadow_mmu(struct kvm *kvm)
+{
+ kvm->arch.shadow_mmu_active = true;
+}
+
I think there's no lock protecting both the write and the read side.
Therefore this should be an smp_store_release, and all checks in
patch 2 should be an smp_load_acquire.
That makes sense.
Also, the assignments to slot->arch.rmap in patch 4 (alloc_memslot_rmap)
should be an rcu_assign_pointer, while __gfn_to_rmap must be changed like so:
+ struct kvm_rmap_head *head;
...
- return &slot->arch.rmap[level - PG_LEVEL_4K][idx];
+ head = srcu_dereference(slot->arch.rmap[level - PG_LEVEL_4K], &kvm->srcu,
+ lockdep_is_held(&kvm->slots_arch_lock));
+ return &head[idx];
I'm not sure I fully understand why this becomes necessary after patch
4. Isn't it already needed since the memslots are protected by RCU? Or
is there already a higher level rcu dereference?
__kvm_memslots already does an srcu dereference, so is there a path
where we aren't getting the slots from that function where this is
needed?
There are two point of views:
1) the easier one is just CONFIG_PROVE_RCU debugging: the rmaps need to
be accessed under RCU because the memslots can disappear as soon as
kvm->srcu is unlocked.
2) the harder one (though at this point I'm better at figuring out these
ordering bugs than "traditional" mutex races) is what the happens before
relation[1] looks like. Consider what happens if the rmaps are
allocated by *another thread* after the slots have been fetched.
thread 1 thread 2 thread 3
allocate memslots
rcu_assign_pointer
slots = srcu_dereference
allocate rmap
rcu_assign_pointer
head = slot->arch.rmap[]
Here, thread 3 is allocating the rmaps in the SRCU-protected
kvm_memslots; those rmaps that didn't exist at the time thread 1 did the
rcu_assign_pointer (which synchronizes with thread 2's srcu_dereference
that retrieves slots), hence they were not covered by the release
semantics of that rcu_assign_pointer and the "consume" semantics of the
corresponding srcu_dereference. Therefore, thread 2 needs another
srcu_dereference when retrieving them.
Paolo
[1] https://lwn.net/Articles/844224/
I wouldn't say that the rmaps are protected by RCU in any way that
separate from the memslots.
