> -----Original Message----- > From: Xie, Huawei > Sent: Monday, March 21, 2016 5:26 PM > To: Ananyev, Konstantin; Ilya Maximets; Yuanhan Liu > Cc: dev at dpdk.org; Dyasly Sergey > Subject: Re: [dpdk-dev] [PATCH v4] vhost: use SMP barriers instead of > compiler ones. > > On 3/21/2016 10:07 PM, Ananyev, Konstantin wrote: > > > >> -----Original Message----- > >> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Ilya Maximets > >> Sent: Monday, March 21, 2016 4:50 AM > >> To: Yuanhan Liu > >> Cc: dev at dpdk.org; Xie, Huawei; Dyasly Sergey > >> Subject: Re: [dpdk-dev] [PATCH v4] vhost: use SMP barriers instead of > >> compiler ones. > >> > >> > >> > >> On 18.03.2016 15:41, Yuanhan Liu wrote: > >>> On Fri, Mar 18, 2016 at 03:23:53PM +0300, Ilya Maximets wrote: > >>>> Since commit 4c02e453cc62 ("eal: introduce SMP memory barriers") virtio > >>>> uses architecture dependent SMP barriers. vHost should use them too. > >>>> > >>>> Fixes: 4c02e453cc62 ("eal: introduce SMP memory barriers") > >>>> > >>>> Signed-off-by: Ilya Maximets <i.maximets at samsung.com> > >>>> --- > >>>> lib/librte_vhost/vhost_rxtx.c | 7 ++++--- > >>>> 1 file changed, 4 insertions(+), 3 deletions(-) > >>>> > >>>> diff --git a/lib/librte_vhost/vhost_rxtx.c > >>>> b/lib/librte_vhost/vhost_rxtx.c > >>>> index b4da665..859c669 100644 > >>>> --- a/lib/librte_vhost/vhost_rxtx.c > >>>> +++ b/lib/librte_vhost/vhost_rxtx.c > >>>> @@ -315,7 +315,7 @@ virtio_dev_rx(struct virtio_net *dev, uint16_t > >>>> queue_id, > >>>> rte_prefetch0(&vq->desc[desc_indexes[i+1]]); > >>>> } > >>>> > >>>> - rte_compiler_barrier(); > >>>> + rte_smp_wmb(); > >>>> > >>>> /* Wait until it's our turn to add our buffer to the used ring. > >>>> */ > >>>> while (unlikely(vq->last_used_idx != res_start_idx)) > >>>> @@ -565,7 +565,7 @@ virtio_dev_merge_rx(struct virtio_net *dev, uint16_t > >>>> queue_id, > >>>> > >>>> nr_used = copy_mbuf_to_desc_mergeable(dev, vq, start, > >>>> end, > >>>> pkts[pkt_idx]); > >>>> - rte_compiler_barrier(); > >>>> + rte_smp_wmb(); > >>>> > >>>> /* > >>>> * Wait until it's our turn to add our buffer > >>>> @@ -923,7 +923,8 @@ rte_vhost_dequeue_burst(struct virtio_net *dev, > >>>> uint16_t queue_id, > >>>> sizeof(vq->used->ring[used_idx])); > >>>> } > >>>> > >>>> - rte_compiler_barrier(); > >>>> + rte_smp_wmb(); > >>>> + rte_smp_rmb(); > >>> rte_smp_mb? > >> rte_smp_mb() is a real mm_fence() on x86. And we don't need to synchronize > >> reads with > >> writes here, only reads with reads and writes with writes. It is enough > >> because next > >> increment uses read and write. Pair of barriers is better because it will > >> not impact > >> on performance on x86. > > Not arguing about that particular patch, just a question: > > Why do we have: > > #define rte_smp_mb() rte_mb() > > Konstantine, actually smp_mb is defined as mfence while smp_r/wmb are > defined as barrier in kernel_src/arch/x86/include/asm/barrier.h.
I am aware of that, but who said that we should do the same? Konstantin > > > for x86? > > Why not just: > > #define rte_smp_mb() rte_compiler_barrier() > > here? > > I meant for situations when we do need real mfence, there is an 'rte_mb' to > > use. > > Konstantin > > > >> Best regards, Ilya Maximets.