From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:50062) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1XK55K-0000j1-33 for qemu-devel@nongnu.org; Wed, 20 Aug 2014 08:33:54 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1XK55C-0001Gr-GX for qemu-devel@nongnu.org; Wed, 20 Aug 2014 08:33:50 -0400 Received: from mx1.redhat.com ([209.132.183.28]:30034) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1XK55C-0001Gl-93 for qemu-devel@nongnu.org; Wed, 20 Aug 2014 08:33:42 -0400 Date: Wed, 20 Aug 2014 14:33:57 +0200 From: "Michael S. Tsirkin" Message-ID: <20140820123357.GA18304@redhat.com> References: <1408517593.25437.102.camel@ori.omang.mine.nu> <53F461E6.5020506@redhat.com> <1408527055.14053.107.camel@abi.no.oracle.com> <87k3631i29.fsf@blackfin.pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87k3631i29.fsf@blackfin.pond.sub.org> Subject: Re: [Qemu-devel] [PATCH 2/3] ioh3420: Provide a unique bus name and an interrupt mapping function List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Markus Armbruster Cc: Marcel Apfelbaum , Alexey Kardashevskiy , Juan Quintela , Knut Omang , qemu-devel@nongnu.org, Gonglei , Igor Mammedov , Paolo Bonzini On Wed, Aug 20, 2014 at 02:06:38PM +0200, Markus Armbruster wrote: > Knut Omang writes: > > > On Wed, 2014-08-20 at 10:52 +0200, Paolo Bonzini wrote: > >> Il 20/08/2014 08:53, Knut Omang ha scritto: > >> > A unique bus name is necessary to be able to refer to each instance > >> > from the command line and monitors. > >> > >> Is it needed? Can't you just add id= to the -device option? > > > > Yes, as far as I understand the problem is that the id= would work on > > the ioh3420 device itself, while what is needed here is to name the > > secondary bus of the ioh3420, which I haven't found a way to name from > > the command line. > > Bus names in qdev are a mess. Here are the rules: > > 1. If code provides a name, that's the name. > > 2. Else, if the device has an ID, the name is ID.N, where N counts the > device's buses from zero. > > 3. Else, the name is BUS-TYPE-NAME.N, where N counts the buses of this > type from zero. > > This results in a usable bus name unless device IDs collide with bus > type names, or the code provides names that collide. > > The user needs to take care to use IDs that don't collide with bus > names. Adding new bus names may screw some users. > > The user needs to take further care to use IDs whenever the code > provides a bus name that collides. Adding code that provides bus names > may screw some users. > > Broken by design. > > The problem here is "code provides names that collide": device > q35-pcihost provides the name "pcie.0". Bound to collide with the first > PCIE bus named under rule 3. For instance, if you next add an ioh3420 > without ID, its bus is also named "pcie.0". > > Rule 1 should be taken out and shot. Unfortunately, that'll break ABI > left & right. Instead, we can try to reduce its use. The appended does > exactly that for q35-pcihost. With it applied, the bus provided by > q35-pcihost still gets the same name "pcie.0", but under rule 3 instead > of rule 1. Rule 3 then names further PCIE buses "pcie.1", "pcie.2", ... > instead of "pcie.0", "pcie.1", ... Better, but it's still an ABI break. > > > Maybe an even better solution would be to have default names for > > everything, if not specified, from a user friendliness perspective? > > Buses *have* a default name! You're confusing this with device IDs, > which exist only when the user sets one. > > Changes in this area are difficult, because the names are all ABI. > Names that cannot be used are fair game, of course. > > > I suppose this is a more general issue of sensible default values > > though, but the fact that it is easy to create devices which cannot be > > referred has caused me some confusion from time to time. > > Picking default qdev IDs risks collisions with the user's IDs. We > shouldn't do that. We do it anyway in a few places, for historical > reasons. > > QOM paths might be a sane way to let users refer to devices without IDs. > > > While writing the above, I stumbled another rule 1 screwup: pci_bridge.c > attempts to "improve" the boring standard bus names chosen via rule 2 or > 3. > > pci_bridge_initfn() provides a bus name of its own (commit 8a3d80f > pci_bridge: user-friendly default bus name): > a. If pci_bridge_map_irq() set a bus name, that's the name. > > b. Else, if the device has an ID, that's the name. Thus, ID.N is > "improved" to just ID, at the cost of a special case: now users have to > avoid not just IDs of the form BUS-TYPE-NAME.N, but also plain > BUS-TYPE-NAME. > > Callers of pci_bridge_map_irq() generally provide a name. Some names > contain spaces, thus can't collide (but would be bloody inconvenient on > the command line or in the monitor). Others don't, but thankfully the > ones I checked are in dead code. Craptastic. > > > diff --git a/hw/pci-host/q35.c b/hw/pci-host/q35.c > index 37f228e..469aafd 100644 > --- a/hw/pci-host/q35.c > +++ b/hw/pci-host/q35.c > @@ -47,7 +47,7 @@ static void q35_host_realize(DeviceState *dev, Error **errp) > sysbus_add_io(sbd, MCH_HOST_BRIDGE_CONFIG_DATA, &pci->data_mem); > sysbus_init_ioports(sbd, MCH_HOST_BRIDGE_CONFIG_DATA, 4); > > - pci->bus = pci_bus_new(DEVICE(s), "pcie.0", > + pci->bus = pci_bus_new(DEVICE(s), NULL, > s->mch.pci_address_space, s->mch.address_space_io, > 0, TYPE_PCIE_BUS); > qdev_set_parent_bus(DEVICE(&s->mch), BUS(pci->bus)); This is for the root bus, I think it won't help Knut who's trying to add devices behind root ports.