linux-kernel - Re: [Xen-devel] [PATCH 6/7] xen/9pfs: receive responses

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Message-ID: <20170308201157.GA9643@char.us.oracle.com>
Date:   Wed, 8 Mar 2017 15:11:57 -0500
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     Stefano Stabellini <sstabellini@...nel.org>
Cc:     jgross@...e.com, Latchesar Ionkov <lucho@...kov.net>,
        Eric Van Hensbergen <ericvh@...il.com>,
        linux-kernel@...r.kernel.org,
        Stefano Stabellini <stefano@...reto.com>,
        v9fs-developer@...ts.sourceforge.net,
        Ron Minnich <rminnich@...dia.gov>,
        xen-devel@...ts.xenproject.org,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [Xen-devel] [PATCH 6/7] xen/9pfs: receive responses

On Tue, Mar 07, 2017 at 05:13:59PM -0800, Stefano Stabellini wrote:
> On Tue, 7 Mar 2017, Stefano Stabellini wrote:
> > > > +
> > > > +	ring = container_of(work, struct xen_9pfs_dataring, work);
> > > > +	priv = ring->priv;
> > > > +
> > > > +	while (1) {
> > > > +		cons = ring->intf->in_cons;
> > > > +		prod = ring->intf->in_prod;
> > > > +		rmb();
> > > 
> > > 
> > > Is this rmb() or mb()? (Or, in fact, virt_XXX()?) You used mb() in the
> > > previous patch.
> >  
> > I think they should all be virt_XXX, thanks.
> 
> regarding mb() vs. rmb(), give a look at the workflow at the end of
> docs/misc/9pfs.markdown, under "Ring Usage".

That is not what Boris meant. He meant that you should use the
virt_ variants instead of the rmb() or wmb().

The reason that on UP kernels the rmb() and wmb() can be converted
to NOPs. While that is OK for a UP kernel it is not good in virtualization
as we need those barriers regardless of the flavor of the kernel.

> 
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