From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:55901) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1XK5ZT-0002Xq-ES for qemu-devel@nongnu.org; Wed, 20 Aug 2014 09:05:04 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1XK5ZO-0002eC-Ov for qemu-devel@nongnu.org; Wed, 20 Aug 2014 09:04:59 -0400 Received: from mx1.redhat.com ([209.132.183.28]:32269) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1XK5ZO-0002e5-Gc for qemu-devel@nongnu.org; Wed, 20 Aug 2014 09:04:54 -0400 From: Markus Armbruster 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> <20140820123357.GA18304@redhat.com> Date: Wed, 20 Aug 2014 15:03:59 +0200 In-Reply-To: <20140820123357.GA18304@redhat.com> (Michael S. Tsirkin's message of "Wed, 20 Aug 2014 14:33:57 +0200") Message-ID: <87ppfvxqgw.fsf@blackfin.pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain 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: "Michael S. Tsirkin" Cc: Juan Quintela , Alexey Kardashevskiy , Marcel Apfelbaum , Knut Omang , qemu-devel@nongnu.org, Gonglei , Paolo Bonzini , Igor Mammedov "Michael S. Tsirkin" writes: > 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. Read again, more slowly :) Yes, I null the name of the root bus. That makes the qdev machinery derive the very same "pcie.0" name via rule 3 instead of rule 1, with the side effect that future (non-root) PCIE buses get different names. In particular, the next one named via rule 3 will be called "pcie.1" instead of "pcie.0", making it actually accessible.