On Fri, Jan 29, 2010 at 07:44:59AM +1030, Rusty Russell wrote:
> On Fri, 29 Jan 2010 12:07:08 am Michael S. Tsirkin wrote:
> > On Thu, Jan 28, 2010 at 10:01:09AM +1030, Rusty Russell wrote:
> > > On Thu, 28 Jan 2010 09:12:23 am Michael S. Tsirkin wrote:
> > > > Where possible, we should use SMP barriers which are more lightweight
> > > > than
> > > > mandatory barriers, because mandatory barriers also control MMIO
> > > > effects on
> > > > accesses through relaxed memory I/O windows (which virtio does not use)
> > > > (compare specifically smp_rmb and rmb on x86_64).
> > >
> > > Xen had a similar issue, in that UP guests running on SMP hosts need to
> > > issue
> > > SMP barriers. Is this not also a requirement for virtio?
> >
> > Of course it is. That's why I have ifdef CONFIG_SMP and use
> > mandatory barriers on UP.
>
> Sorry, this was an off-the-cuff comment. I was concerned that UP barriers
> might be insufficient on some archs. But I can't find any such archs when
> I actually look (at least, x86, s390 and powerpc).
>
> I've applied your patch, minus the first two substitutions:
>
> @@ -36,10 +54,10 @@
> panic("%s:in_use = %i\n", \
> (_vq)->vq.name, (_vq)->in_use); \
> (_vq)->in_use = __LINE__; \
> - mb(); \
> + virtio_mb(); \
> } while (0)
> #define END_USE(_vq) \
> - do { BUG_ON(!(_vq)->in_use); (_vq)->in_use = 0; mb(); } while(0)
> + do { BUG_ON(!(_vq)->in_use); (_vq)->in_use = 0; virtio_mb(); }
> while(0)
> #else
> #define BAD_RING(_vq, fmt, args...) \
> do { \
>
> These barriers are actually trying to make sure in_use is set in a timely
> manner (this is debug code). They're bogus AFAICT: if you don't have any
> other synchronization then you're in danger of nesting and you want to know.
> And barriers might change timing too much when DEBUG is defined.
>
> Thanks!
> Rusty.
So .. let's just drop these barriers, and replace in_use with an atomic?
--
MST
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