From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:47597) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aHH7g-0005HD-Ao for qemu-devel@nongnu.org; Thu, 07 Jan 2016 15:25:29 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1aHH7c-0008CH-A3 for qemu-devel@nongnu.org; Thu, 07 Jan 2016 15:25:28 -0500 Received: from mx1.redhat.com ([209.132.183.28]:39003) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aHH7c-0008Bt-4J for qemu-devel@nongnu.org; Thu, 07 Jan 2016 15:25:24 -0500 References: <20160107110747.10897.41118.stgit@bahia.huguette.org> <20160107113232.10897.40641.stgit@bahia.huguette.org> From: Laurent Vivier Message-ID: <568EC9AF.20107@redhat.com> Date: Thu, 7 Jan 2016 21:25:19 +0100 MIME-Version: 1.0 In-Reply-To: <20160107113232.10897.40641.stgit@bahia.huguette.org> Content-Type: text/plain; charset=utf-8 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: Greg Kurz , "Michael S. Tsirkin" Cc: qemu-devel@nongnu.org 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