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Re: [Qemu-devel] [PATCH] virtio: Make memory barriers be memory barriers

From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] [PATCH] virtio: Make memory barriers be memory barriers
Date: Tue, 6 Sep 2011 12:35:14 +0300
User-agent: Mutt/1.5.21 (2010-09-15)

On Tue, Sep 06, 2011 at 12:28:40PM +0300, Avi Kivity wrote:
> On 09/06/2011 09:55 AM, Paolo Bonzini wrote:
> >On 09/06/2011 05:12 AM, David Gibson wrote:
> >>I'm not "fixing ppc".  I'm fixing a fundamental flaw in the protocol
> >>implementation._So far_  I've only observed the effects on ppc, but
> >>that doesn't mean they don't exist.
> >
> >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.

Yes but the macros in question are here to order qemu accesses,
not guest accesses.
> Although in that case the guest is probably required to issue an SFENCE.
> -- 
> error compiling committee.c: too many arguments to function

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