On 8/9/22 16:44, Sean Christopherson wrote:
On Tue, Aug 09, 2022, Paolo Bonzini wrote:
On 8/9/22 05:26, Yan Zhao wrote:
hi Sean,
I understand this smp_rmb() is intended to prevent the reading of
p->nx_huge_page_disallowed from happening before it's set to true in
kvm_tdp_mmu_map(). Is this understanding right?
If it's true, then do we also need the smp_rmb() for read of sp->gfn in
handle_removed_pt()? (or maybe for other fields in sp in other places?)
No, in that case the barrier is provided by rcu_dereference(). In fact, I
am not sure the barriers are needed in this patch either (but the comments
are :)):
Yeah, I'm 99% certain the barriers aren't strictly required, but I didn't love the
idea of depending on other implementation details for the barriers. Of course I
completely overlooked the fact that all other sp fields would need the same
barriers...
- the write barrier is certainly not needed because it is implicit in
tdp_mmu_set_spte_atomic's cmpxchg64
- the read barrier _should_ also be provided by rcu_dereference(pt), but I'm
not 100% sure about that. The reasoning is that you have
(1) iter->old spte = READ_ONCE(*rcu_dereference(iter->sptep));
...
(2) tdp_ptep_t pt = spte_to_child_pt(old_spte, level);
(3) struct kvm_mmu_page *sp = sptep_to_sp(rcu_dereference(pt));
...
(4) if (sp->nx_huge_page_disallowed) {
and (4) is definitely ordered after (1) thanks to the READ_ONCE hidden
within (3) and the data dependency from old_spte to sp.
Yes, I think that's correct. Callers must verify the SPTE is present before getting
the associated child shadow page. KVM does have instances where a shadow page is
retrieved from the SPTE _pointer_, but that's the parent shadow page, i.e. isn't
guarded by the SPTE being present.
struct kvm_mmu_page *sp = sptep_to_sp(rcu_dereference(iter->sptep));
Something like this is as a separate patch?
Would you resubmit without the memory barriers then?
diff --git a/arch/x86/kvm/mmu/tdp_iter.h b/arch/x86/kvm/mmu/tdp_iter.h
index f0af385c56e0..9d982ccf4567 100644
--- a/arch/x86/kvm/mmu/tdp_iter.h
+++ b/arch/x86/kvm/mmu/tdp_iter.h
@@ -13,6 +13,12 @@
* to be zapped while holding mmu_lock for read, and to allow TLB flushes to be
* batched without having to collect the list of zapped SPs. Flows that can
* remove SPs must service pending TLB flushes prior to dropping RCU protection.
+ *
+ * The READ_ONCE() ensures that, if the SPTE points at a child shadow page, all
+ * fields in struct kvm_mmu_page will be read after the caller observes the
+ * present SPTE (KVM must check that the SPTE is present before following the
+ * SPTE's pfn to its associated shadow page). Pairs with the implicit memory
I guess you mean both the shadow page table itself and the struct
kvm_mmu_page? Or do you think to_shadow_page() should have a smp_rmb()?
Paolo
