Re: [PATCH RFC 3/4] barriers: convert a control to a data dependency

[Peter Zijlstra]

On Mon, Jan 07, 2019 at 08:36:36AM -0500, Michael S. Tsirkin wrote:
> On Mon, Jan 07, 2019 at 10:46:10AM +0100, Peter Zijlstra wrote:

> > How about naming the thing: dependent_ptr() ? That is without any (r)mb
> > implications at all. The address dependency is strictly weaker than an
> > rmb in that it will only order the two loads in qestion and not, like
> > rmb, any prior to any later load.
> 
> So I'm fine with this as it's enough for virtio, but I would like to point out two things:
> 
> 1. E.g. on x86 both SMP and DMA variants can be NOPs but
> the madatory one can't, so assuming we do not want
> it to be stronger than rmp then either we want
> smp_dependent_ptr(), dma_dependent_ptr(), dependent_ptr()
> or we just will specify that dependent_ptr() works for
> both DMA and SMP.

The latter; the construct simply generates dependent loads. It is up to
the CPU as to what all that works for.

> 2. Down the road, someone might want to order a store after a load.
> Address dependency does that for us too. Assuming we make
> dependent_ptr a NOP on x86, we will want an mb variant
> which isn't a NOP on x86. Will we want to rename
> dependent_ptr to dependent_ptr_rmb at that point?

Not sure; what is the actual overhead of the construct on x86 vs the
NOP?