netdev - Re: [PATCH 0/3] virtio/vringh: kill off ACCESS

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Date:   Fri, 25 Nov 2016 11:22:03 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Christian Borntraeger <borntraeger@...ibm.com>
Cc:     linux-kernel@...r.kernel.org, dave@...olabs.net, dbueso@...e.de,
        dvyukov@...gle.com, jasowang@...hat.com, kvm@...r.kernel.org,
        netdev@...r.kernel.org, paulmck@...ux.vnet.ibm.com,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE()

On Thu, Nov 24, 2016 at 10:36:58PM +0200, Michael S. Tsirkin wrote:
> On Thu, Nov 24, 2016 at 10:25:11AM +0000, Mark Rutland wrote:
> > For several reasons, it would be beneficial to kill off ACCESS_ONCE()
> > tree-wide, in favour of {READ,WRITE}_ONCE(). These work with aggregate types,
> > more obviously document their intended behaviour, and are necessary for tools
> > like KTSAN to work correctly (as otherwise reads and writes cannot be
> > instrumented separately).
> > 
> > While it's possible to script the bulk of this tree-wide conversion, some cases
> > such as the virtio code, require some manual intervention. This series moves
> > the virtio and vringh code over to {READ,WRITE}_ONCE(), in the process fixing a
> > bug in the virtio headers.
> > 
> > Thanks,
> > Mark.
> 
> I don't have a problem with this specific patchset.

Good to hear. :)

Does that mean you're happy to queue these patches? Or would you prefer
a new posting at some later point, with ack/review tags accumulated?

> Though I really question the whole _ONCE APIs esp with
> aggregate types - these seem to generate a memcpy and
> an 8-byte read/writes sometimes, and I'm pretty sure this simply
> can't be read/written at once on all architectures.

Yes, in cases where the access is larger than the machine can perform in
a single access, this will result in a memcpy.

My understanding is that this has always been the case with
ACCESS_ONCE(), where multiple accesses were silently/implicitly
generated by the compiler.

We could add some compile-time warnings for those cases. I'm not sure if
there's a reason we avoided doing that so far; perhaps Christian has a
some idea.

Thanks,
Mark.