From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752314AbdI0W5Y (ORCPT ); Wed, 27 Sep 2017 18:57:24 -0400 Received: from mx1.redhat.com ([209.132.183.28]:36732 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751671AbdI0W5W (ORCPT ); Wed, 27 Sep 2017 18:57:22 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com 3E1A9117AA9 Authentication-Results: ext-mx06.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com Authentication-Results: ext-mx06.extmail.prod.ext.phx2.redhat.com; spf=fail smtp.mailfrom=mst@redhat.com Date: Thu, 28 Sep 2017 01:57:20 +0300 From: "Michael S. Tsirkin" To: Jason Wang Cc: virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org Subject: Re: [PATCH net-next RFC 2/5] vhost: introduce helper to prefetch desc index Message-ID: <20170928012906-mutt-send-email-mst@kernel.org> References: <1506067355-5771-1-git-send-email-jasowang@redhat.com> <1506067355-5771-3-git-send-email-jasowang@redhat.com> <20170926221435-mutt-send-email-mst@kernel.org> <17e9c3a9-7759-a674-bc00-414eabfed118@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <17e9c3a9-7759-a674-bc00-414eabfed118@redhat.com> X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.30]); Wed, 27 Sep 2017 22:57:22 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Sep 27, 2017 at 08:35:47AM +0800, Jason Wang wrote: > > > On 2017年09月27日 03:19, Michael S. Tsirkin wrote: > > On Fri, Sep 22, 2017 at 04:02:32PM +0800, Jason Wang wrote: > > > This patch introduces vhost_prefetch_desc_indices() which could batch > > > descriptor indices fetching and used ring updating. This intends to > > > reduce the cache misses of indices fetching and updating and reduce > > > cache line bounce when virtqueue is almost full. copy_to_user() was > > > used in order to benefit from modern cpus that support fast string > > > copy. Batched virtqueue processing will be the first user. > > > > > > Signed-off-by: Jason Wang > > > --- > > > drivers/vhost/vhost.c | 55 +++++++++++++++++++++++++++++++++++++++++++++++++++ > > > drivers/vhost/vhost.h | 3 +++ > > > 2 files changed, 58 insertions(+) > > > > > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c > > > index f87ec75..8424166d 100644 > > > --- a/drivers/vhost/vhost.c > > > +++ b/drivers/vhost/vhost.c > > > @@ -2437,6 +2437,61 @@ struct vhost_msg_node *vhost_dequeue_msg(struct vhost_dev *dev, > > > } > > > EXPORT_SYMBOL_GPL(vhost_dequeue_msg); > > > +int vhost_prefetch_desc_indices(struct vhost_virtqueue *vq, > > > + struct vring_used_elem *heads, > > > + u16 num, bool used_update) > > why do you need to combine used update with prefetch? > > For better performance Why is sticking a branch in there better than requesting the update conditionally from the caller? > and I believe we don't care about the overhead when > we meet errors in tx. That's a separate question, I do not really understand how you can fetch a descriptor and update the used ring at the same time. This allows the guest to overwrite the buffer. I might be misunderstanding what is going on here though. > > > > > +{ > > > + int ret, ret2; > > > + u16 last_avail_idx, last_used_idx, total, copied; > > > + __virtio16 avail_idx; > > > + struct vring_used_elem __user *used; > > > + int i; > > > + > > > + if (unlikely(vhost_get_avail(vq, avail_idx, &vq->avail->idx))) { > > > + vq_err(vq, "Failed to access avail idx at %p\n", > > > + &vq->avail->idx); > > > + return -EFAULT; > > > + } > > > + last_avail_idx = vq->last_avail_idx & (vq->num - 1); > > > + vq->avail_idx = vhost16_to_cpu(vq, avail_idx); > > > + total = vq->avail_idx - vq->last_avail_idx; > > > + ret = total = min(total, num); > > > + > > > + for (i = 0; i < ret; i++) { > > > + ret2 = vhost_get_avail(vq, heads[i].id, > > > + &vq->avail->ring[last_avail_idx]); > > > + if (unlikely(ret2)) { > > > + vq_err(vq, "Failed to get descriptors\n"); > > > + return -EFAULT; > > > + } > > > + last_avail_idx = (last_avail_idx + 1) & (vq->num - 1); > > > + } > > > + > > > + if (!used_update) > > > + return ret; > > > + > > > + last_used_idx = vq->last_used_idx & (vq->num - 1); > > > + while (total) { > > > + copied = min((u16)(vq->num - last_used_idx), total); > > > + ret2 = vhost_copy_to_user(vq, > > > + &vq->used->ring[last_used_idx], > > > + &heads[ret - total], > > > + copied * sizeof(*used)); > > > + > > > + if (unlikely(ret2)) { > > > + vq_err(vq, "Failed to update used ring!\n"); > > > + return -EFAULT; > > > + } > > > + > > > + last_used_idx = 0; > > > + total -= copied; > > > + } > > > + > > > + /* Only get avail ring entries after they have been exposed by guest. */ > > > + smp_rmb(); > > Barrier before return is a very confusing API. I guess it's designed to > > be used in a specific way to make it necessary - but what is it? > > Looks like a and we need do this after reading avail_idx. > > Thanks > > > > > > > > + return ret; > > > +} > > > +EXPORT_SYMBOL(vhost_prefetch_desc_indices); > > > static int __init vhost_init(void) > > > { > > > diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h > > > index 39ff897..16c2cb6 100644 > > > --- a/drivers/vhost/vhost.h > > > +++ b/drivers/vhost/vhost.h > > > @@ -228,6 +228,9 @@ ssize_t vhost_chr_read_iter(struct vhost_dev *dev, struct iov_iter *to, > > > ssize_t vhost_chr_write_iter(struct vhost_dev *dev, > > > struct iov_iter *from); > > > int vhost_init_device_iotlb(struct vhost_dev *d, bool enabled); > > > +int vhost_prefetch_desc_indices(struct vhost_virtqueue *vq, > > > + struct vring_used_elem *heads, > > > + u16 num, bool used_update); > > > #define vq_err(vq, fmt, ...) do { \ > > > pr_debug(pr_fmt(fmt), ##__VA_ARGS__); \ > > > -- > > > 2.7.4