Messages in this thread | | | From | Paolo Bonzini <> | Subject | Re: [PATCH v2 1/7] KVM: x86/mmu: Track if shadow MMU active | Date | Tue, 4 May 2021 22:18:40 +0200 |
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On 04/05/21 19:26, Ben Gardon wrote: > On Mon, May 3, 2021 at 6:42 AM Paolo Bonzini <pbonzini@redhat.com> 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.
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