[libvirt] [PATCH 03/12] vcpu: add new public API

[Eric Blake]

On 10/01/2010 10:13 AM, Matthias Bolte wrote:
>> However, in implementing things, I'm wondering if I should use the names:
>>
>> VIR_DOMAIN_VCPU_CONFIG (instead of VIR_DOMAIN_VCPU_PERSISTENT)
>> VIR_DOMAIN_VCPU_LIVE (instead of VIR_DOMAIN_VCPU_ACTIVE)
>>
>> to match virDomainDeviceModifyFlags, where _CONFIG and _LIVE have
>> the same semantics of setting one or both aspects of a domain.
>>
>
> If you want to match the semantics of _CONFIG and _LIVE from
> virDomainDeviceModifyFlags then you need to change the ESX driver
> differently in patch 7/12.
>
> If I understand the semantics of virDomainDeviceModifyFlags correctly,
> then _LIVE is only affecting the runtime state of the domain and
> changes done with the _LIVE flag only are lost on domain restart. In
> the same way _CONFIG only affects the persistent config and not the
> runtime state.

Yes, that was the usage pattern I had envisioned, to allow for domains 
like qemu where you can make a live-only change.

>
> With ESX there is no such thing as a distinct runtime state. Changes
> to the number of vCPUs affect the runtime state and the persistent
> config at the same time.

And, in playing with things a bit more, this is what I see pre-patch on 
Fedora 14 (# for host, $ for guest):

# virsh edit fedora_14
# virsh dumpxml fedora_14 | grep vcpu
   <vcpu>2</vcpu>
# virsh start fedora_14
Domain fedora_14 started
# virsh setvcpus fedora_14 3
[hmm, trying to exceed the max killed my guest - not very nice]
# virsh setvcpus fedora_14 1
error: Requested operation is not valid: domain is not running
[it currently requires a live domain]
# virsh dumpxml fedora_14 | grep vcpu
   <vcpu>3</vcpu>
[but it changed the configured value]

# virsh start fedora_14
Domain fedora_14 started
# virsh setvcpus fedora_14 2
# virsh dumpxml fedora_14 | grep vcpu
   <vcpu>2</vcpu>

$ nproc
3
[hmm - the live value is still 3]
$ shutdown -h now

# virsh start fedora_14
Domain fedora_14 started

$ nproc
2
[yep - definitely a persistent setting, and I couldn't figure how it is 
supposed to be live, since I could not see qemuMonitorSetCPU having a 
visible effect on nproc(1) in the guest?]


>
> Currently you altered the ESX driver in patch 7/12 to
>
>    esxDomainSetVcpus = esxDomainSetVcpusFlags(VIR_DOMAIN_VCPU_ACTIVE)
>
> But this should actually use _LIVE | _CONFIG as flags to match what
> ESX can do only.

I'm starting to think that should be the case in general: 
virDomainSetVcpus should map to 
virDomainSetVcpusFlags(VIR_DOMAIN_VCPU_LIVE|VIR_DOMAIN_VCPU_CONFIG), and 
the new behavior becomes that of specifying live-only or config-only.

There may yet be some logic bugs for some drivers, and certainly this 
argues for additions to the TCK on hot-plugged vcpu behavior.  At any 
rate, I really need to post a v2 series that tries to be more careful 
with my handling of live vs. configuration.

>
> Also in 5/12 you state that trying to change the vCPU count of an
> inactive domain using the _LIVE flag should fail. The implementation
> in the ESX driver in 7/12 doesn't match this either.

If the domain is inactive, then _LIVE can't possibly change anything.  I 
could probably agree that it makes more sense to have _CONFIG|_LIVE 
silently ignore _LIVE when a domain is inactive rather than erroring 
out, especially given the argument that the existing setVcpus command 
without flags should probably be using _CONFIG|_LIVE; except that the 
qemu example above proved that the existing setVcpus errors out if the 
domain is not live.  And I still think that _LIVE in isolation on an 
inactive domain should error out (the _CONFIG|_LIVE combination is 
different, because _CONFIG can still do useful work).

>
> esxDomainSetVcpus needs to call esxDomainSetVcpusFlags with _CONFIG
> and add _LIVE to the set of flags when the domain is active. Likewise
> esxDomainSetVcpusFlags needs to enforce that _CONFIG is always set and
> that _LIVE is set iff the domain is active.
>
> The same might apply to other drivers, I didn't check this yet.
>
> Matthias
>

-- 
Eric Blake   eblake at redhat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org