On Tue, Nov 29, 2011 at 03:57:19PM +0200, Ohad Ben-Cohen wrote: > On Tue, Nov 29, 2011 at 3:11 PM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: > > On Tue, Nov 29, 2011 at 02:31:26PM +0200, Ohad Ben-Cohen wrote: > >> Virtio is using memory barriers to control the ordering of > >> references to the vrings on SMP systems. When the guest is compiled > >> with SMP support, virtio is only using SMP barriers in order to > >> avoid incurring the overhead involved with mandatory barriers. > >> > >> Lately, though, virtio is being increasingly used with inter-processor > >> communication scenarios too, which involve running two (separate) > >> instances of operating systems on two (separate) processors, each of > >> which might either be UP or SMP. > > > > Is that using virtio-mmio? > > No, I'm using this: > > https://lkml.org/lkml/2011/10/25/139 This mentions iommu - is there a need to use dma api to let the firmware acess the rings? Or does it have access to all of memory? > > Sorry, could you pls explain what are 'two external processors'? > > I think I know that if two x86 CPUs in an SMP system run kernels built > > in an SMP configuration, smp_*mb barriers are enough. > > Sure: > > My setup is not SMP-based; it's two separate processors running in AMP > configuration. The processors have completely different architectures, > are not cache coherent, and only simply share some memory, which is > used for communications using virtio as the shared memory "wire" > protocol (i.e. we're not even doing virtualization: we have Linux on > one processor, and some RTOS on another processor, and they use virtio > to send and receive buffers). I'd like to make sure I understand the memory model some more. Is there cache snooping? If yes access from an external device typically works mostly in the same way as smp ... > So it's not SMP effects we're controlling; we're pretty much doing > MMIO and must use mandatory barriers So you put virtio rings in MMIO memory? > (otherwise we see breakage). Could you please give a couple of examples of breakage? -- MST _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
