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Date: Tue, 06 Sep 2011 11:38:59 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] virtio: Make memory barriers be memory
	barriers
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To: Avi Kivity <avi@redhat.com>
Cc: aliguori@us.ibm.com, "Michael S. Tsirkin" <mst@redhat.com>, aik@ozlabs.ru, rusty@rustcorp.com.au, agraf@suse.de, qemu-devel@nongnu.org

On 09/06/2011 11:28 AM, Avi Kivity wrote:
>>
>> Actually Michael is right.  The implementation is correct on x86,
>> though wrong anywhere else (perhaps s390?).  On those architectures
>> you do not need rmb() and wmb().
>
> Are we sure?  Nothing prevents the guest from using weakly-ordered
> writes, is there?  For example, using MOVNTDQ.
>
> Although in that case the guest is probably required to issue an SFENCE.

Yes, that's the guest problem.  You cannot do an SFENCE on the guest's 
behalf anyway.

Paolo