From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756929AbbFPVMG (ORCPT ); Tue, 16 Jun 2015 17:12:06 -0400 Received: from mx1.redhat.com ([209.132.183.28]:41925 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753325AbbFPVL5 (ORCPT ); Tue, 16 Jun 2015 17:11:57 -0400 Date: Tue, 16 Jun 2015 23:11:54 +0200 From: "Michael S. Tsirkin" To: Igor Mammedov Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, pbonzini@redhat.com Subject: Re: [PATCH 5/5] vhost: translate_desc: optimization for desc.len < region size Message-ID: <20150616230957-mutt-send-email-mst@redhat.com> References: <1434472419-148742-1-git-send-email-imammedo@redhat.com> <1434472419-148742-6-git-send-email-imammedo@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1434472419-148742-6-git-send-email-imammedo@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 16, 2015 at 06:33:39PM +0200, Igor Mammedov wrote: > when translating descriptors they are typically less than > memory region that holds them and translated into 1 iov > enty, entry > so it's not nessesary to check remaining length > twice and calculate used length and next address > in such cases. > > so relace replace > a remaining length and 'size' increment branches > with a single remaining length check and execute > next iov steps only when it needed. it is needed > > It saves tiny 2% of translate_desc() execution time. a tiny > > Signed-off-by: Igor Mammedov > --- > PS: > I'm not sure if iov_size > 0 is always true, if it's not > then better to drop this patch. > --- > drivers/vhost/vhost.c | 21 +++++++++++++-------- > 1 file changed, 13 insertions(+), 8 deletions(-) > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c > index 68c1c88..84c457d 100644 > --- a/drivers/vhost/vhost.c > +++ b/drivers/vhost/vhost.c > @@ -1111,12 +1111,8 @@ static int translate_desc(struct vhost_virtqueue *vq, u64 addr, u32 len, > int ret = 0; > > mem = vq->memory; > - while ((u64)len > s) { > + do { > u64 size; > - if (unlikely(ret >= iov_size)) { > - ret = -ENOBUFS; > - break; > - } > reg = find_region(mem, addr, len, &vq->cached_reg); > if (unlikely(!reg)) { > ret = -EFAULT; > @@ -1124,13 +1120,22 @@ static int translate_desc(struct vhost_virtqueue *vq, u64 addr, u32 len, > } > _iov = iov + ret; > size = reg->memory_size - addr + reg->guest_phys_addr; > - _iov->iov_len = min((u64)len - s, size); > _iov->iov_base = (void __user *)(unsigned long) > (reg->userspace_addr + addr - reg->guest_phys_addr); > + ++ret; > + if (likely((u64)len - s < size)) { > + _iov->iov_len = (u64)len - s; > + break; > + } > + > + if (unlikely(ret >= iov_size)) { > + ret = -ENOBUFS; > + break; > + } > + _iov->iov_len = size; > s += size; > addr += size; > - ++ret; > - } > + } while (1); > > return ret; > } > -- > 1.8.3.1