[libvirt] [Qemu-devel] [PATCH v7 1/3] qmp: adding 'wakeup-suspend-support' in query-target
Markus Armbruster
armbru at redhat.com
Mon May 28 07:23:54 UTC 2018
Eduardo Habkost <ehabkost at redhat.com> writes:
> On Fri, May 25, 2018 at 08:30:59AM +0200, Markus Armbruster wrote:
>> Eduardo Habkost <ehabkost at redhat.com> 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.
The duplication is less than elegant, but backward-compatibility
generally is.
>> 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.
Analyzing all the use cases we know makes sense.
> [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")) {
How can we get from this grep to a list of static or dynamic machine
type capabilties?
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