From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751147AbcL1IJm (ORCPT ); Wed, 28 Dec 2016 03:09:42 -0500 Received: from mx1.redhat.com ([209.132.183.28]:54642 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751092AbcL1IJj (ORCPT ); Wed, 28 Dec 2016 03:09:39 -0500 From: Jason Wang To: mst@redhat.com, kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org Cc: Jason Wang Subject: [PATCH net-next V2 1/3] vhost: better detection of available buffers Date: Wed, 28 Dec 2016 16:09:29 +0800 Message-Id: <1482912571-3157-2-git-send-email-jasowang@redhat.com> In-Reply-To: <1482912571-3157-1-git-send-email-jasowang@redhat.com> References: <1482912571-3157-1-git-send-email-jasowang@redhat.com> X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.39]); Wed, 28 Dec 2016 08:09:38 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This patch tries to do several tweaks on vhost_vq_avail_empty() for a better performance: - check cached avail index first which could avoid userspace memory access. - using unlikely() for the failure of userspace access - check vq->last_avail_idx instead of cached avail index as the last step. This patch is need for batching supports which needs to peek whether or not there's still available buffers in the ring. Signed-off-by: Jason Wang --- drivers/vhost/vhost.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c index d643260..9f11838 100644 --- a/drivers/vhost/vhost.c +++ b/drivers/vhost/vhost.c @@ -2241,11 +2241,15 @@ bool vhost_vq_avail_empty(struct vhost_dev *dev, struct vhost_virtqueue *vq) __virtio16 avail_idx; int r; + if (vq->avail_idx != vq->last_avail_idx) + return false; + r = vhost_get_user(vq, avail_idx, &vq->avail->idx); - if (r) + if (unlikely(r)) return false; + vq->avail_idx = vhost16_to_cpu(vq, avail_idx); - return vhost16_to_cpu(vq, avail_idx) == vq->avail_idx; + return vq->avail_idx == vq->last_avail_idx; } EXPORT_SYMBOL_GPL(vhost_vq_avail_empty); -- 2.7.4