On Mon, Jan 07, 2019 at 04:54:23PM +0100, Peter Zijlstra wrote: > 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. But not on intel right? On intel loads are ordered so it can be a nop. > > 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? I'll have to check. There's a pipeline stall almost for sure - that's why we put it there after all :). -- MST
