From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: [PATCH] virtio_ring: use smp_store_mb Date: Thu, 17 Dec 2015 12:32:53 +0200 Message-ID: <1450347932-16325-1-git-send-email-mst__46919.7600767531$1450348391$gmane$org@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: linux-kernel@vger.kernel.org Cc: qemu-devel@nongnu.org, virtualization@lists.linux-foundation.org List-Id: virtualization@lists.linuxfoundation.org We need a full barrier after writing out event index, using smp_store_mb there seems better than open-coding. As usual, we need a wrapper to account for strong barriers/non smp. It's tempting to use this in vhost as well, for that, we'll need a variant of smp_store_mb that works on __user pointers. Signed-off-by: Michael S. Tsirkin --- Seems to give a speedup on my box but I'm less sure about this one. E.g. as xchng faster than mfence on all/most intel CPUs? Anyone has an opinion? include/linux/virtio_ring.h | 14 ++++++++++++++ drivers/virtio/virtio_ring.c | 15 +++++++++------ 2 files changed, 23 insertions(+), 6 deletions(-) diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h index 0135c16..8912189 100644 --- a/include/linux/virtio_ring.h +++ b/include/linux/virtio_ring.h @@ -47,6 +47,20 @@ static inline void virtio_wmb(bool weak_barriers) wmb(); } +static inline void virtio_store_mb(bool weak_barriers, + __virtio16 *p, __virtio16 v) +{ +#ifdef CONFIG_SMP + if (weak_barriers) + smp_store_mb(*p, v); + else +#endif + { + WRITE_ONCE(*p, v); + mb(); + } +} + static inline __virtio16 virtio_load_acquire(bool weak_barriers, __virtio16 *p) { if (!weak_barriers) { diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c index f822cab..b0aea67 100644 --- a/drivers/virtio/virtio_ring.c +++ b/drivers/virtio/virtio_ring.c @@ -517,10 +517,10 @@ void *virtqueue_get_buf(struct virtqueue *_vq, unsigned int *len) /* If we expect an interrupt for the next entry, tell host * by writing event index and flush out the write before * the read in the next get_buf call. */ - if (!(vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)) { - vring_used_event(&vq->vring) = cpu_to_virtio16(_vq->vdev, vq->last_used_idx); - virtio_mb(vq->weak_barriers); - } + if (!(vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)) + virtio_store_mb(vq->weak_barriers, + &vring_used_event(&vq->vring), + cpu_to_virtio16(_vq->vdev, vq->last_used_idx)); #ifdef DEBUG vq->last_add_time_valid = false; @@ -653,8 +653,11 @@ bool virtqueue_enable_cb_delayed(struct virtqueue *_vq) } /* TODO: tune this threshold */ bufs = (u16)(vq->avail_idx_shadow - vq->last_used_idx) * 3 / 4; - vring_used_event(&vq->vring) = cpu_to_virtio16(_vq->vdev, vq->last_used_idx + bufs); - virtio_mb(vq->weak_barriers); + + virtio_store_mb(vq->weak_barriers, + &vring_used_event(&vq->vring), + cpu_to_virtio16(_vq->vdev, vq->last_used_idx + bufs)); + if (unlikely((u16)(virtio16_to_cpu(_vq->vdev, vq->vring.used->idx) - vq->last_used_idx) > bufs)) { END_USE(vq); return false; -- MST