Re: [libvirt] [Qemu-devel] [PATCH v7 1/3] qmp: adding 'wakeup-suspend-support' in query-target

On Fri, May 25, 2018 at 08:30:59AM +0200, Markus Armbruster wrote: > Eduardo Habkost <ehabkost(a)redhat.com&gt; writes: > > On Wed, May 23, 2018 at 05:53:34PM +0200, Markus Armbruster wrote: [...] > >> >> Worse, a machine type property that is static for all machine types now > >> >> could conceivably become dynamic when we add a machine type > >> >> configuration knob. > >> >> > >> > > >> > This isn't the first time a machine capability that seems static > >> > actually depends on other configuration arguments. We will > >> > probably need to address this eventually. > >> > >> Then the best time to address it is now, provided we can :) > > > > I'm not sure this is the best time. If libvirt only needs the > > runtime value and don't need any info at query-machines time, I > > think support for this on query-machines will be left unused and > > they will only use the query-current-machine value. > > > > Just giving libvirt the runtime data it wants > > (query-current-machine) seems way better than requiring libvirt > > to interpret a set of rules and independently calculate something > > QEMU already knows. > > I wouldn't mind adding a query-current-machine to report dynamic machine > capabilities if that helps QMP clients. query-machines could continue > to report static machine capabilities then. > > However, we do need a plan on how to distribute machine capabilities > between query-machines and query-current-machine, in particular how to > handle changing staticness. Handling dynamic data that becomes static is easy: we can keep it on both.

> wakeup-suspend-support is static for most machine types, but dynamic for > some. Where should it go? I think it obviously should go on query-current-machine. Maybe it can be added to query-machines in the future if it's deemed useful. > It needs to go into query-current-machine when its dynamic with the > current machine. It may go there just to keep things regular even if > its static with the current machine. > > Does it go into query-machines, too? If not, clients lose the ability > to examine all machines efficiently. Even if this isn't an issue for > wakeup-suspend-support: are we sure this can't be an issue for any > future capabilities? I don't think this will be an issue for wakup-suspend-support, but this is already an issue for existing capabilities. See below[1]. > > If it goes into query-machines, what should its value be for the machine > types where it's dynamic? Should it be absent, perhaps, letting clients > know they have to consult query-current-machine to find the value? > > What if a capability previously thought static becomes dynamic? We can > add it to query-current-machine just fine, but removing it from > query-machines would be a compatibility break. Making it optional, > too. Should all new members of MachineInfo be optional, just in case? > The above are questions we must ponder if we are considering extending query-machines. We have been avoiding them for a few years, by simply not extending query-machines very often and letting libvirt hardcode machine-type info. :( > These are design questions we need to ponder *now*. Picking a solution > that satisfies current needs while ignoring future needs has bitten us > in the posterior time and again. We're not going to successfully > predict *all* future needs, but not even trying should be easy to beat. > > That's what I meant by "the best time to address it is now". > I agree. I just think there are countless other use cases that require extending query-machines today. I'd prefer to use one of them as a starting point for this design exercise, instead of wakeup-suspend-support.

[1] Doing a: $ git grep 'STR.*machine, "' on libvirt source is enough to find some code demonstrating where query-machines is already lacking today: src/libxl/libxl_capabilities.c:583: if (STREQ(machine, "xenpv")) src/libxl/libxl_capabilities.c:729: if (STREQ(domCaps->machine, "xenfv")) src/libxl/libxl_driver.c:6379: if (STRNEQ(machine, "xenpv") && STRNEQ(machine, "xenfv")) { src/qemu/qemu_capabilities.c:2435: STREQ(def->os.machine, "ppce500")) src/qemu/qemu_capabilities.c:2439: STREQ(def->os.machine, "prep")) src/qemu/qemu_capabilities.c:2443: STREQ(def->os.machine, "bamboo")) src/qemu/qemu_capabilities.c:2446: if (STREQ(def->os.machine, "mpc8544ds")) src/qemu/qemu_capabilities.c:5376: STREQ(def->os.machine, "isapc"); src/qemu/qemu_command.c:3829: if (STRPREFIX(def->os.machine, "s390-virtio") && src/qemu/qemu_domain.c:2798: if (STREQ(def->os.machine, "isapc")) { src/qemu/qemu_domain.c:5009: if (STREQ(def->os.machine, "versatilepb")) src/qemu/qemu_domain.c:8222: return (STRPREFIX(machine, "pc-q35") || src/qemu/qemu_domain.c:8223: STREQ(machine, "q35")); src/qemu/qemu_domain.c:8237: return (STREQ(machine, "pc") || src/qemu/qemu_domain.c:8238: STRPREFIX(machine, "pc-0.") || src/qemu/qemu_domain.c:8239: STRPREFIX(machine, "pc-1.") || src/qemu/qemu_domain.c:8240: STRPREFIX(machine, "pc-i440") || src/qemu/qemu_domain.c:8241: STRPREFIX(machine, "rhel")); src/qemu/qemu_domain.c:8285: const char *p = STRSKIP(machine, "pc-q35-"); src/qemu/qemu_domain.c:8310: return STRPREFIX(machine, "s390-ccw"); src/qemu/qemu_domain.c:8329: if (STRNEQ(machine, "virt") && src/qemu/qemu_domain.c:8330: !STRPREFIX(machine, "virt-")) src/qemu/qemu_domain.c:8351: if (STRNEQ(machine, "pseries") && src/qemu/qemu_domain.c:8352: !STRPREFIX(machine, "pseries-")) src/qemu/qemu_domain.c:8564: STREQ(machine, "malta") || src/qemu/qemu_domain.c:8565: STREQ(machine, "sun4u") || src/qemu/qemu_domain.c:8566: STREQ(machine, "g3beige"); src/qemu/qemu_domain_address.c:467: if (!(STRPREFIX(def->os.machine, "vexpress-") || src/qemu/qemu_domain_address.c:2145: if (STREQ(def->os.machine, "versatilepb")) src/util/virarch.c:170: } else if (STREQ(ut.machine, "amd64")) {