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

[Eduardo Habkost]

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.

> 
> 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")) {

-- 
Eduardo