[libvirt] [PATCH 03/11] qemu: Pass QEMUCaps to virDomainDefPostParse

[Marc Hartmayer]

On Mon, Oct 01, 2018 at 12:36 PM +0200, Peter Krempa <pkrempa at redhat.com> wrote:

[…snip…]

>> I would do all the precondition stuff (e.g. the priv->qemuCaps isn’t
>> valid anymore) as early as possible in the overall “task/job” process.
>
> Well, technically priv->qemuCaps is invalid and unneeded when the VM is
> not running and valid and immutable if it is running.
>
> If the VM is not running, there is no qemu process associated to it so
> it does not make sense to store random qemuCaps along with it, since the
> qemu binary might change at the point when we attempt to start it.

Yep, that makes sense.

>
> I was not a fan of needing qemuCaps in the post parse infrastructure of
> qemu, but at this point we can't do much about it especially as we want
> to keep the config stable after defined.
>
>> > This patch does not seem to make sense with the justification provided
>> > here as the post-parse callbacks fill in the proper caps here and this
>> > part clearly uses a new domain configuration, so the default behaviour
>> > should be used.
>> >
>> > Changing this would also need that we change the normal parser path to
>> > achieve the same results which is impossible as you don't have access to
>> > priv->qemuCaps prior to creating the virDomainObj object.
>>
>> Yep, that’s why I said in the cover letter “With this patch series the
>> behavior is still not (completely) fixed, but the performance has been
>> significantly improved.”.
>
> I don't see what should be fixed here, but mostly as I did not read the
> series.

The problem is that we currently don’t take into account that the QEMU
binary can change during a task (e.g. define a domain). Therefore, it’s
possible that two calls of virQEMUCapsCacheLookup return different QEMU
capabilities.

[…snip]

--
Kind regards / Beste Grüße
   Marc Hartmayer

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