Re: [PATCH v2 2/3] xenfb: move xen_rmb to the correct location

From: Stefano Stabellini <sstabellini@kernel.org>
To: Wei Liu <wei.liu2@citrix.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	qemu-devel@nongnu.org, David Vrabel <david.vrabel@citrix.com>,
	Anthony Perard <anthony.perard@citrix.com>,
	Xen-devel <xen-devel@lists.xenproject.org>
Subject: Re: [PATCH v2 2/3] xenfb: move xen_rmb to the correct location
Date: Tue, 12 Apr 2016 10:31:29 -0700 (PDT)	[thread overview]
Message-ID: <alpine.DEB.2.10.1604121030060.23347__40210.0579267882$1460482369$gmane$org@sstabellini-ThinkPad-X260> (raw)
In-Reply-To: <20160412134603.GC18652@citrix.com>

On Tue, 12 Apr 2016, Wei Liu wrote:
> On Tue, Apr 12, 2016 at 02:38:13PM +0100, Andrew Cooper wrote:
> > On 12/04/16 13:57, David Vrabel wrote:
> > > On 12/04/16 11:43, Wei Liu wrote:
> > >> It should be placed before first time producer and consumer are used.
> > > This change isn't necessary and is confusing as this is not what this
> > > barrier is for.
> > >
> > > The barrier needs to be between the load of prod and the load of the
> > > ring contents (there's even a comment that says this).  This pairs with
> > > the corresponding write barrier between the store of the ring contents
> > > and the store of prod (in the other end).
> > 
> > Looking further, this code will compile to multiple reads of the page,
> > because there is no ACCESS_ONCE().  This code is still vulnerable to
> > XSA-155.

There is no ACCESS_ONCE in QEMU, the closest thing to it is atomic_read.

> Oops, accidentally kicked over a can of worms. Should have just sent
> patch 1. :-)
> 
> Jokes aside, more time is needed to fix this properly. So maybe we
> should just upstream patch #1 first. Stefano? Anthony?

Sure

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