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Re: [PATCH v2] KVM: dirty ring: add missing memory barrier


From: Gavin Shan
Subject: Re: [PATCH v2] KVM: dirty ring: add missing memory barrier
Date: Wed, 7 Sep 2022 10:37:25 +1000
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.0

On 8/27/22 6:22 PM, Paolo Bonzini wrote:
The KVM_DIRTY_GFN_F_DIRTY flag ensures that the entry is valid.  If
the read of the fields are not ordered after the read of the flag,
QEMU might see stale values.

Cc: Peter Xu <peterx@redhat.com>
Cc: Gavin Shan <gshan@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
  accel/kvm/kvm-all.c | 6 +++++-
  1 file changed, 5 insertions(+), 1 deletion(-)


Reviewed-by: Gavin Shan <gshan@redhat.com>

diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index 8d81ab74de..136c8eaed3 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -719,7 +719,11 @@ static void kvm_dirty_ring_mark_page(KVMState *s, uint32_t 
as_id,
static bool dirty_gfn_is_dirtied(struct kvm_dirty_gfn *gfn)
  {
-    return gfn->flags == KVM_DIRTY_GFN_F_DIRTY;
+    /*
+     * Read the flags before the value.  Pairs with barrier in
+     * KVM's kvm_dirty_ring_push() function.
+     */
+    return qatomic_load_acquire(&gfn->flags) == KVM_DIRTY_GFN_F_DIRTY;
  }
static void dirty_gfn_set_collected(struct kvm_dirty_gfn *gfn)





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