[Qemu-devel] Re: [PATCH RFC] virtio: put last seen used index into ring itself

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, May 07, 2010 at 01:03:28PM +0930, Rusty Russell wrote:
> On Thu, 6 May 2010 03:49:46 pm Michael S. Tsirkin wrote:
> > Now, I also added an mb() in guest between read and write so
> > that last used index write can not get ahead of used index read.
> > It does feel good to have it there, but I can not say why
> > it's helpful. Works fine without it, but then these
> > subtle races might be hard to trigger. What do you think?
> 
> I couldn't see that in the patch?  I don't think it's necessary
> though, since the write of depends last_used depends on the read of
> used (and no platform we care about would reorder such a thing).

Well, there's no data dependency, is there?

> I'm reasonably happy, but we should write some convenient test for
> missing interrupts.
> 
> I'm thinking of a sender which does a loop: blasts 1MB of UDP packets,
> then prints the time and sleep(1).  The receiver would print the time
> every 1MB of received data.  The two times should almost exactly correspond.
> 
> Assuming that the network doesn't overflow and lose stuff, this should
> identify any missing wakeup/interrupts (depending on direction used).
> 
> Cheers,
> Rusty.