Re: [Qemu-devel] [PATCH] virtio_ring: use smp_store_mb

[Peter Zijlstra]

On Thu, Dec 17, 2015 at 03:16:20PM +0200, Michael S. Tsirkin wrote:
> On Thu, Dec 17, 2015 at 11:52:38AM +0100, Peter Zijlstra wrote:
> > On Thu, Dec 17, 2015 at 12:32:53PM +0200, Michael S. Tsirkin wrote:
> > > +static inline void virtio_store_mb(bool weak_barriers,
> > > +                            __virtio16 *p, __virtio16 v)
> > > +{
> > > +#ifdef CONFIG_SMP
> > > + if (weak_barriers)
> > > +         smp_store_mb(*p, v);
> > > + else
> > > +#endif
> > > + {
> > > +         WRITE_ONCE(*p, v);
> > > +         mb();
> > > + }
> > > +}
> > 
> > This is a different barrier depending on SMP, that seems wrong.
> 
> Of course it's wrong in the sense that it's
> suboptimal on UP. What we would really like is to
> have, on UP, exactly the same barrier as on SMP.
> This is because a UP guest can run on an SMP host.
> 
> But Linux doesn't provide this ability: if CONFIG_SMP is
> not defined is optimizes most barriers out to a
> compiler barrier.
> 
> Consider for example x86: what we want is xchg (NOT
> mfence - see below for why) but if built without CONFIG_SMP
> smp_store_mb does not include this.

You could of course go fix that instead of mutilating things into
sort-of functional state.

> 
> 
> > smp_mb(), as (should be) used by smp_store_mb() does not provide a
> > barrier against IO. mb() otoh does.
> > 
> > Since this is virtIO I would expect you always want mb().
> 
> No because it's VIRTio not real io :) It just switches to the hyprevisor
> mode - kind of like a function call really.
> The weak_barriers flag is cleared for when it's used
> with real devices with real IO.
> 
> 
> All this is explained in some detail at the top of
> include/linux/virtio.h

I did read that, it didn't make any sense wrt the code below it.

For instance it seems to imply weak_barriers is for smp like stuff while
!weak_barriers is for actual devices.

But then you go use dma_*mb() ops, which are specifially for devices
only for weak_barrier.