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

From: Xiao Guangrong
Date: Thu Mar 10 2016 - 09:41:14 EST

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

Quote from Avi:
| I don't think we need to flush immediately; set a "tlb dirty" bit
| that is cleareded when we flush the tlb.
| 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();
@@ -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();
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.
+ */
if (kvm_make_all_cpus_request(kvm, KVM_REQ_TLB_FLUSH))
cmpxchg(&kvm->tlbs_dirty, dirty_count, 0);

Any comment?