From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:58963) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aHTKW-0006HM-FF for qemu-devel@nongnu.org; Fri, 08 Jan 2016 04:27:33 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1aHTKS-0003c9-7q for qemu-devel@nongnu.org; Fri, 08 Jan 2016 04:27:32 -0500 Received: from e06smtp15.uk.ibm.com ([195.75.94.111]:59777) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aHTKR-0003c3-V9 for qemu-devel@nongnu.org; Fri, 08 Jan 2016 04:27:28 -0500 Received: from localhost by e06smtp15.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Fri, 8 Jan 2016 09:27:27 -0000 Received: from b06cxnps4076.portsmouth.uk.ibm.com (d06relay13.portsmouth.uk.ibm.com [9.149.109.198]) by d06dlp02.portsmouth.uk.ibm.com (Postfix) with ESMTP id 936422190046 for ; Fri, 8 Jan 2016 09:27:13 +0000 (GMT) Received: from d06av11.portsmouth.uk.ibm.com (d06av11.portsmouth.uk.ibm.com [9.149.37.252]) by b06cxnps4076.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id u089RO8454460456 for ; Fri, 8 Jan 2016 09:27:24 GMT Received: from d06av11.portsmouth.uk.ibm.com (localhost [127.0.0.1]) by d06av11.portsmouth.uk.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id u089ROh2031177 for ; Fri, 8 Jan 2016 02:27:24 -0700 Date: Fri, 8 Jan 2016 10:27:20 +0100 From: Greg Kurz Message-ID: <20160108102720.67883872@bahia.local> In-Reply-To: <568EC9AF.20107@redhat.com> References: <20160107110747.10897.41118.stgit@bahia.huguette.org> <20160107113232.10897.40641.stgit@bahia.huguette.org> <568EC9AF.20107@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH 6/6] virtio: optimize virtio_access_is_big_endian() for little-endian targets List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Laurent Vivier Cc: qemu-devel@nongnu.org, "Michael S. Tsirkin" On Thu, 7 Jan 2016 21:25:19 +0100 Laurent Vivier wrote: > > > On 07/01/2016 12:32, Greg Kurz wrote: > > When adding cross-endian support, we introduced the TARGET_IS_BIENDIAN macro > > and the virtio_access_is_big_endian() helper to have a branchless fast path > > in the virtio memory accessors for targets that don't switch endian. > > > > This was considered as a strong requirement at the time. > > > > Now we have added a runtime check for virtio 1.0, which ruins the benefit > > of the virtio_access_is_big_endian() helper for always little-endian targets. > > > > With this patch, always little-endian targets stop checking for virtio 1.0, > > since the result is little-endian in all cases. > > > > Signed-off-by: Greg Kurz > > --- > > include/hw/virtio/virtio-access.h | 3 +++ > > 1 file changed, 3 insertions(+) > > > > diff --git a/include/hw/virtio/virtio-access.h b/include/hw/virtio/virtio-access.h > > index f1f12afe9089..fba4b4a4e1b2 100644 > > --- a/include/hw/virtio/virtio-access.h > > +++ b/include/hw/virtio/virtio-access.h > > @@ -19,10 +19,13 @@ > > > > static inline bool virtio_access_is_big_endian(VirtIODevice *vdev) > > { > > +#if defined(TARGET_IS_BIENDIAN) || defined(TARGET_WORDS_BIGENDIAN) > > if (virtio_vdev_has_feature(vdev, VIRTIO_F_VERSION_1)) { > > /* Devices conforming to VIRTIO 1.0 or later are always LE. */ > > return false; > > } > > +#endif > > > virtio_is_big_endian() checks VIRTIO_F_VERSION_1, so you don't need to > check it here if TARGET_IS_BIENDIAN is defined. > > > + > > #if defined(TARGET_IS_BIENDIAN) > > return virtio_is_big_endian(vdev); > > #elif defined(TARGET_WORDS_BIGENDIAN) > > > > > So you can move virtio_vdev_has_feature() here. > > Laurent > > Indeed this is one step further and results in: static inline bool virtio_access_is_big_endian(VirtIODevice *vdev) { +#if defined(TARGET_IS_BIENDIAN) + return virtio_is_big_endian(vdev); +#elif defined(TARGET_WORDS_BIGENDIAN) if (virtio_vdev_has_feature(vdev, VIRTIO_F_VERSION_1)) { /* Devices conforming to VIRTIO 1.0 or later are always LE. */ return false; } -#if defined(TARGET_IS_BIENDIAN) - return virtio_is_big_endian(vdev); -#elif defined(TARGET_WORDS_BIGENDIAN) return true; #else return false; which looks nicer: slowest path (2 branches) for bi-endian targets, slow path (1 branch) for big endian targets and fast path for little endian targets. Thanks for the suggestion ! -- Greg