From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:36534) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TsKGP-0004fU-2r for qemu-devel@nongnu.org; Mon, 07 Jan 2013 16:29:47 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1TsKGM-0003Bi-MJ for qemu-devel@nongnu.org; Mon, 07 Jan 2013 16:29:45 -0500 Received: from mx1.redhat.com ([209.132.183.28]:37555) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TsKGM-0003B9-Dc for qemu-devel@nongnu.org; Mon, 07 Jan 2013 16:29:42 -0500 Date: Mon, 7 Jan 2013 23:33:20 +0200 From: "Michael S. Tsirkin" Message-ID: <20130107213320.GA11305@redhat.com> References: <1357584074-10852-1-git-send-email-fred.konrad@greensocs.com> <1357584074-10852-20-git-send-email-fred.konrad@greensocs.com> <20130107211611.GE8679@redhat.com> <87hams1yn5.fsf@codemonkey.ws> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87hams1yn5.fsf@codemonkey.ws> Subject: Re: [Qemu-devel] [PATCH 19/61] virtio-net : cleanup : use QOM cast. List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Anthony Liguori Cc: kwolf@redhat.com, peter.maydell@linaro.org, e.voevodin@samsung.com, mark.burton@greensocs.com, qemu-devel@nongnu.org, agraf@suse.de, cornelia.huck@de.ibm.com, aneesh.kumar@linux.vnet.ibm.com, stefanha@redhat.com, amit.shah@redhat.com, pbonzini@redhat.com, afaerber@suse.de, fred.konrad@greensocs.com On Mon, Jan 07, 2013 at 03:17:18PM -0600, Anthony Liguori wrote: > "Michael S. Tsirkin" writes: > > > On Mon, Jan 07, 2013 at 07:40:32PM +0100, fred.konrad@greensocs.com wrote: > >> @@ -130,7 +124,9 @@ static void virtio_net_set_status(struct VirtIODevice *vdev, uint8_t status) > >> > >> static void virtio_net_set_link_status(NetClientState *nc) > >> { > >> - VirtIONet *n = DO_UPCAST(NICState, nc, nc)->opaque; > >> + void *opaque = DO_UPCAST(NICState, nc, nc)->opaque; > >> + VirtIONet *n = VIRTIO_NET(opaque); > >> + VirtIODevice *vdev = VIRTIO_DEVICE(n); > >> uint16_t old_status = n->status; > >> > >> if (nc->link_down) > > > > I note this adds more pointer chasing due to runtime casts on data path > > operations. Can well be trivial but this really needs to be verified > > with a performance test. Was this done? Same comment applies to block. > > An alternative is to add _fast casts without runtime checks. > > I'm pretty sure other things like the one big global mutex are more > important than the number of pointer dereferences... True. But we have plans to fix that, right? > Do you have any evidence to suggest that this would be a problem in practice? > > Regards, > > Anthony Liguori No. It's not this change specifically, I'm generally concerned if we let ourselves bloat datapath in an uncontrolled way, the waste will add up over time. > > > > -- > > MST