On Wed, Nov 30, 2011 at 06:04:56PM +0200, Ohad Ben-Cohen wrote: > On Wed, Nov 30, 2011 at 4:59 PM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: > > I see. And this happens because the ARM processor reorders > > memory writes > > Yes. > > > And in an SMP configuration, writes are somehow not reordered? > > They are, but then the smp memory barriers are enough to control these > effects. It's not enough to control reordering as seen by a device > (which is what our AMP processors are) though. > > (btw, the difference between an SMP processor and a device here lies > in how the memory is mapped: normal memory vs. device memory > attributes. it's an ARM thingy). How are the rings mapped? normal memory, right? We allocate them with plan alloc_pages_exact in virtio_pci.c ... > > Just checking that this is not a bug in the smp_wmb implementation > > for the specific platform. > > No, it's not. > > ARM's smp memory barriers use ARM's DMB instruction, which is enough > to control SMP effects, whereas ARM's mandatory memory barriers use > ARM's DSB instruction, which is required to ensure the ordering > between Device and Normal memory accesses. > > Thanks, > Ohad. Yes wmb() is required to ensure ordering for MMIO. But here both accesses: index and ring - are for memory, not MMIO. I could understand ring kick bypassing index write, maybe ... But you described an index write bypassing descriptor write. Is this something you see in practice? -- MST -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
