Re: [PATCH] KVM: Remove redundant smp_mb() in the kvm_mmu_commit_zap_page()

[Xiao Guangrong]

On 03/08/2016 11:27 PM, Paolo Bonzini wrote:
> On 08/03/2016 09:36, Lan Tianyu wrote:
> > Summary about smp_mb()s we met in this thread. If misunderstood, please
> > correct me. Thanks.
> >
> > The smp_mb() in the kvm_flush_remote_tlbs() was introduced by the commit
> > a4ee1ca4 and it seems to keep the order of reading and cmpxchg
> > kvm->tlbs_dirty.
> >
> >      Quote from Avi:
> >      | I don't think we need to flush immediately; set a "tlb dirty" bit
> > somewhere
> >      | that is cleareded when we flush the tlb.
> > kvm_mmu_notifier_invalidate_page()
> >      | can consult the bit and force a flush if set.
> >
> >      Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxx>
> >      Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
>
> Unfortunately that patch added a bad memory barrier: 1) it lacks a
> comment; 2) it lacks obvious pairing; 3) it is an smp_mb() after a read,
> so it's not even obvious that this memory barrier has to do with the
> immediately preceding read of kvm->tlbs_dirty.  It also is not
> documented in Documentation/virtual/kvm/mmu.txt (Guangrong documented
> there most of his other work, back in 2013, but not this one :)).
>
> The cmpxchg is ordered anyway against the read, because 1) x86 has
> implicit ordering between earlier loads and later stores; 2) even
> store-load barriers are unnecessary for accesses to the same variable
> (in this case kvm->tlbs_dirty).
>
> So offhand, I cannot say what it orders.  There are two possibilities:
>
> 1) it orders the read of tlbs_dirty with the read of mode.  In this
> case, a smp_rmb() would have been enough, and it's not clear where is
> the matching smp_wmb().
>
> 2) it orders the read of tlbs_dirty with the KVM_REQ_TLB_FLUSH request.
>   In this case a smp_load_acquire would be better.
>
> 3) it does the same as kvm_mmu_commit_zap_page's smp_mb() but for other
> callers of kvm_flush_remote_tlbs().  In this case, we know what's the
> matching memory barrier (walk_shadow_page_lockless_*).
>
> 4) it is completely unnecessary.

Sorry, memory barriers were missed in sync_page(), this diff should fix it:

diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
index 91e939b..4cad57f 100644
--- a/arch/x86/kvm/paging_tmpl.h
+++ b/arch/x86/kvm/paging_tmpl.h
@@ -948,6 +948,12 @@ static int FNAME(sync_page)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
 			return -EINVAL;

 		if (FNAME(prefetch_invalid_gpte)(vcpu, sp, &sp->spt[i], gpte)) {
+			/*
+			 * update spte before increasing tlbs_dirty to make sure no tlb
+			 * flush in lost after spte is zapped, see the comments in
+			 * kvm_flush_remote_tlbs().
+			 */
+			smp_wmb();
 			vcpu->kvm->tlbs_dirty++;
 			continue;
 		}
@@ -963,6 +969,8 @@ static int FNAME(sync_page)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)

 		if (gfn != sp->gfns[i]) {
 			drop_spte(vcpu->kvm, &sp->spt[i]);
+			/* the same as above where we are doing prefetch_invalid_gpte(). */
+			smp_wmb();
 			vcpu->kvm->tlbs_dirty++;
 			continue;
 		}
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 314c777..82c86ea 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -193,7 +193,12 @@ void kvm_flush_remote_tlbs(struct kvm *kvm)
 {
 	long dirty_count = kvm->tlbs_dirty;

+        /*
+         * read tlbs_dirty before doing tlb flush to make sure not tlb request is
+         * lost.
+         */
 	smp_mb();
+
 	if (kvm_make_all_cpus_request(kvm, KVM_REQ_TLB_FLUSH))
 		++kvm->stat.remote_tlb_flush;
 	cmpxchg(&kvm->tlbs_dirty, dirty_count, 0);


Any comment?