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

[Gavin Shan]

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)