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

[Stefano Stabellini]

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