From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sean Christopherson Subject: Re: [RFC PATCH V3 1/5] vhost: generalize adding used elem Date: Fri, 4 Jan 2019 16:33:39 -0800 Message-ID: <20190105003339.GE11288@linux.intel.com> References: <20181229124656.3900-1-jasowang@redhat.com> <20181229124656.3900-2-jasowang@redhat.com> <20190104162857-mutt-send-email-mst@kernel.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Jason Wang , kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, davem@davemloft.net To: "Michael S. Tsirkin" Return-path: Content-Disposition: inline In-Reply-To: <20190104162857-mutt-send-email-mst@kernel.org> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Fri, Jan 04, 2019 at 04:29:34PM -0500, Michael S. Tsirkin wrote: > On Sat, Dec 29, 2018 at 08:46:52PM +0800, Jason Wang wrote: > > Use one generic vhost_copy_to_user() instead of two dedicated > > accessor. This will simplify the conversion to fine grain > > accessors. About 2% improvement of PPS were seen during vitio-user > > txonly test. > > > > Signed-off-by: Jason Wang > > I don't hve a problem with this patch but do you have > any idea how come removing what's supposed to be > an optimization speeds things up? With SMAP, the 2x vhost_put_user() will also mean an extra STAC/CLAC pair, which is probably slower than the overhead of CALL+RET to whatever flavor of copy_user_generic() gets used. CALL+RET is really the only overhead since all variants of copy_user_generic() unroll accesses smaller than 64 bytes, e.g. on a 64-bit system, __copy_to_user() will write all 8 bytes in a single MOV. Removing the special casing also eliminates a few hundred bytes of code as well as the need for hardware to predict count==1 vs. count>1. > > > --- > > drivers/vhost/vhost.c | 11 +---------- > > 1 file changed, 1 insertion(+), 10 deletions(-) > > > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c > > index 55e5aa662ad5..f179b5ee14c4 100644 > > --- a/drivers/vhost/vhost.c > > +++ b/drivers/vhost/vhost.c > > @@ -2174,16 +2174,7 @@ static int __vhost_add_used_n(struct vhost_virtqueue *vq, > > > > start = vq->last_used_idx & (vq->num - 1); > > used = vq->used->ring + start; > > - if (count == 1) { > > - if (vhost_put_user(vq, heads[0].id, &used->id)) { > > - vq_err(vq, "Failed to write used id"); > > - return -EFAULT; > > - } > > - if (vhost_put_user(vq, heads[0].len, &used->len)) { > > - vq_err(vq, "Failed to write used len"); > > - return -EFAULT; > > - } > > - } else if (vhost_copy_to_user(vq, used, heads, count * sizeof *used)) { > > + if (vhost_copy_to_user(vq, used, heads, count * sizeof *used)) { > > vq_err(vq, "Failed to write used"); > > return -EFAULT; > > } > > -- > > 2.17.1