On Thu, Jan 10, 2019 at 12:55:19PM +0300, Ilya Maximets wrote: > On 10.01.2019 12:18, Maxime Coquelin wrote: > > Hi Gavin, > > > > On 1/10/19 9:19 AM, Gavin Hu (Arm Technology China) wrote: > >> > >> > >>> -----Original Message----- > >>> From: dev <dev-boun...@dpdk.org> On Behalf Of Ilya Maximets > >>> Sent: Wednesday, January 9, 2019 10:50 PM > >>> To: dev@dpdk.org; Maxime Coquelin <maxime.coque...@redhat.com>; > >>> Michael S . Tsirkin <m...@redhat.com>; Xiao Wang > >>> <xiao.w.w...@intel.com> > >>> Cc: Tiwei Bie <tiwei....@intel.com>; Zhihong Wang > >>> <zhihong.w...@intel.com>; jfreim...@redhat.com; Jason Wang > >>> <jasow...@redhat.com>; xiaolong...@intel.com; > >>> alejandro.luc...@netronome.com; Ilya Maximets > >>> <i.maxim...@samsung.com> > >>> Subject: [dpdk-dev] [PATCH v3 2/3] net/virtio: update memory ordering > >>> comment for vq notify > >>> > >>> We're not using io ports in case of modern device even on IA. > >>> Also, this comment useless for other architectures. > >> > >> Agree, it should be architecture neutral. > >> > >>> > >>> Signed-off-by: Ilya Maximets <i.maxim...@samsung.com> > >>> --- > >>> drivers/net/virtio/virtqueue.h | 9 ++++----- > >>> 1 file changed, 4 insertions(+), 5 deletions(-) > >>> > >>> diff --git a/drivers/net/virtio/virtqueue.h > >>> b/drivers/net/virtio/virtqueue.h > >>> index dffa03669..53aeac238 100644 > >>> --- a/drivers/net/virtio/virtqueue.h > >>> +++ b/drivers/net/virtio/virtqueue.h > >>> @@ -437,14 +437,13 @@ virtqueue_kick_prepare_packed(struct virtqueue > >>> *vq) > >>> return flags != RING_EVENT_FLAGS_DISABLE; > >>> } > >>> > >>> +/* > >>> + * virtqueue_kick_prepare*() or the virtio_wmb() should be called > >>> + * before this function to be sure that all the data is visible to vhost. > >>> + */ > >> > >> C11 _atomic APIs are preferred for new code, other than wmb or rmb, could > >> you work on that? > > > > Thanks for the review. > > -rc2 deadline is today, so I may apply this series as is if no reply > > from Ilya today. If a change is agreed, it could be made on top for > > -rc3. > > IMHO, If we'll decide to move to C11 atomics, we'll need to rewrite all > the significant memory accesses in virtio driver at once. We can't do > this partially. This will require significant amount of work to understand > how to do that and will require a lot of testing. So, it's definitely not > for current release. Also, there are possible performance concerns about > such solution. > > But I have even more significant concern: C11 atomics are designed for > inter-thread synchronization on multi-core systems. But in case of vDPA we > have real hardware and I'm not sure if we can use C11 atomics for > cross-domain synchronizations. Do you know if some of the memory ordering > types in C11 provide outer domain sync on ARMv8, for example ?
I would add to that - compiler support might not work well in all versions which can be used to build dpdk. E.g. this article https://lwn.net/Articles/691128/ says that "there will be some seriously suboptimal code production before gcc-7.1". HTH > > > > Regards, > > Maxime > >>> static inline void > >>> virtqueue_notify(struct virtqueue *vq) > >>> { > >>> - /* > >>> - * Ensure updated avail->idx is visible to host. > >>> - * For virtio on IA, the notificaiton is through io port operation > >>> - * which is a serialization instruction itself. > >>> - */ > >>> VTPCI_OPS(vq->hw)->notify_queue(vq->hw, vq); > >>> } > >>> > >>> -- > >>> 2.17.1 > >> > > > >
