On Fri, Sep 14, 2012 at 12:47:52AM -0500, Bryan Venteicher wrote:
> Hi
>
> ----- Original Message -----
> > From: "John Baldwin" <j...@freebsd.org>
> > To: "Bryan Venteicher" <bry...@daemoninthecloset.org>
> > Cc: svn-src-head@freebsd.org, svn-src-...@freebsd.org,
> > src-committ...@freebsd.org, "Peter Grehan"
> > <gre...@freebsd.org>
> > Sent: Thursday, September 13, 2012 1:56:03 PM
> > Subject: Re: svn commit: r240427 - head/sys/dev/virtio
> >
> > On Thursday, September 13, 2012 12:40:42 pm Bryan Venteicher wrote:
> > > > Would it be possible to use atomic_load/store() instead of direct
> > > > memory barriers? For example:
> > > >
> > >
> > > I've been sitting on a (lightly tested) patch [1] for awhile that
> > > does just that, but am not very happy with it. A lot of the fields
> > > are 16-bit, which not all architectures have atomic(9) support for.
> > > And I think the atomic(9) behavior on UP kernels does not provide
> > > the same guarantees as on an SMP kernel (could have an UP kernel
> > > on an SMP host).
> >
> > That is the one thing I was worried about (the fields being defined
> > to be 16-bit). I presume that is required by the virtio de facto
> > standard? Shame we can't clue-by-four people putting 16-bit fields
> > in these sort of things. :-P
> >
>
> Yes, the 16-bit fields are mandated by the VirtIO spec. The guest/host
> shared memory is rounded up to next full page, so there actually isn't
> any memory savings for typical queue sizes. Doubt it is any worse than
> actual hardware regardless.
>
> > > I also found myself wanting an atomic_load_rel_*() type function.
> >
> > That would be odd I think. _rel barriers only affect stores, so
> > there would be no defined ordering between the load and the
> > subsequent stores. (With our current definitions of _acq and
> > _rel.) If you need a full fence for some reason, than a plain
> > mb() may be the best thing in that case.
> >
>
> I'm able to batch add descriptors (via vq_ring_update_avail()),
> but when checking if I must notify the host, I need to make sure
> the latest avail->idx is visible before checking the flag from
> the host on whether notifications are disabled. Gratuitous
> notifications are fine, but skipping one is not.
>
> In the patch, I kludge this with:
> atomic_add_rel_16(&flags, 0);
> foo = flags;
Don't you need
atomic_store_rel_16(&foo, flags);
instead ?You might do a cas_rel over the containing 32bit word as well. > > Hoping the dependency would prevent the assignment to foo from > floating above the atomic_add_rel(). > > I originally did the atomic(9) work just to see if there would be > any performance difference between the two - I wasn't able to > measure any, but I don't have the most modern hardware either. > > Bryan > > > -- > > John Baldwin > >
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