From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:40781) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Xj2Rn-0003Hw-OO for qemu-devel@nongnu.org; Tue, 28 Oct 2014 04:48:16 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Xj2Ri-00069e-Tw for qemu-devel@nongnu.org; Tue, 28 Oct 2014 04:48:11 -0400 Received: from szxga02-in.huawei.com ([119.145.14.65]:11292) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Xj2Ri-000697-85 for qemu-devel@nongnu.org; Tue, 28 Oct 2014 04:48:06 -0400 Message-ID: <544F5835.2070309@huawei.com> Date: Tue, 28 Oct 2014 16:47:49 +0800 From: Bin Wu MIME-Version: 1.0 References: <1414462382-24264-1-git-send-email-wu.wubin@huawei.com> <20141028053228.GD18689@redhat.com> In-Reply-To: <20141028053228.GD18689@redhat.com> Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH] hw/virtio/virtio.c: fix the vring_avail_event error List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Michael S. Tsirkin" Cc: pbonzini@redhat.com, famz@redhat.com, qemu-devel@nongnu.org On 2014/10/28 13:32, Michael S. Tsirkin wrote: > On Tue, Oct 28, 2014 at 02:13:02AM +0000, Bin Wu wrote: >> The event idx in virtio is an effective way to reduce the number of >> interrupts and exits of the guest. When the guest puts an request >> into the virtio ring, it doesn't exit immediately to inform the >> backend. Instead, the guest checks the "avail" event idx to determine >> the notification. >> >> In virtqueue_pop, when a request is poped, the current avail event >> idx should be set to the number of vq->last_avail_idx. >> >> Signed-off-by: Bin Wu > > Does this fix some observable bug? Improve efficiency for some workload? > Improve efficiency, I test as follows: (1)create a suse11sp3 VM with a virtio-blk test disk; (2)use libaio library to submit 32 IOs concurrently (invoke io_sumbmit 32 times) in the VM; (3)modify the virtio-blk driver in the guest to record the host notification times; without this patch, virtio-blk may notify the host between 10 and 30 times; with this patch, virtio-blk just notify the host once during every test:) >> --- >> hw/virtio/virtio.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c >> index 2c236bf..013979a 100644 >> --- a/hw/virtio/virtio.c >> +++ b/hw/virtio/virtio.c >> @@ -469,7 +469,7 @@ int virtqueue_pop(VirtQueue *vq, VirtQueueElement *elem) >> >> i = head = virtqueue_get_head(vq, vq->last_avail_idx++); >> if (vdev->guest_features & (1 << VIRTIO_RING_F_EVENT_IDX)) { >> - vring_avail_event(vq, vring_avail_idx(vq)); >> + vring_avail_event(vq, vq->last_avail_idx); >> } >> >> if (vring_desc_flags(vdev, desc_pa, i) & VRING_DESC_F_INDIRECT) { >> -- >> 1.7.12.4 > > . > -- Bin Wu