Re: [Xen-devel] [PATCH v2 00/20] VM forking

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

On Wed, Jan 8, 2020 at 11:44 AM Roger Pau Monné <roger.pau@xxxxxxxxxx> wrote:
>
> On Wed, Jan 08, 2020 at 11:23:29AM -0700, Tamas K Lengyel wrote:
> > > > > > Why do you need a config file for launching the Qemu device model?
> > > > > > Doesn't the save-file contain all the information?
> > > > >
> > > > > The config is used to populate xenstore, not just for QEMU. The QEMU
> > > > > save file doesn't contain the xl config. This is not a full VM save
> > > > > file, it is only the QEMU state that gets dumped with
> > > > > xen-save-devices-state.
> > > >
> > > > TBH I think it would be easier to have something like my proposal
> > > > below, where you tell xl the parent and the forked VM names and xl
> > > > does the rest. Even better would be to not have to tell xl the parent
> > > > VM name (since I guess this is already tracked internally somewhere?).
> > >
> > > The forked VM has no "name" when it's created. For performance reasons
> > > when the VM fork is created with "--launch-dm no" we explicitly want
> > > to avoid touching Xenstore. Even parsing the config file would be
> > > unneeded overhead at that stage.
> >
> > And to answer your question, no, the parent VM's name is not recorded
> > anywhere for the fork. Technically not even the parent's domain id is
> > kept by Xen. The fork only keeps a pointer to the parent's "struct
> > domain"
>
> There's the domain_id field inside of struct domain, so it seems quite
> easy to get the parent domid from the fork if there's a pointer to the
> parent's struct domain.
>
> > So right now there is no hypercall interface to retrieve a
> > fork's parent's ID - it is assumed the tools using the interface are
> > keeping track of that. Could this information be dumped into Xenstore
> > as well? Yes. But we specifically want be able to create the fork as
> > fast possible without any unnecessary overhead.
>
> I think it would be nice to identify forked domains using
> XEN_DOMCTL_getdomaininfo: you could add a parent_domid field to
> xen_domctl_getdomaininfo and if it's set to something different than
> DOMID_INVALID then the domain is a fork of the given domid.
>
> Not saying it should be done now, but AFAICT getting the parent's
> domid is feasible and doesn't require xenstore.
>

Of course it could be done. I was just pointing out that it's not
currently kept separately and there is no interface to retrieve it.
But TBH I have lost the train the though why we would need that in the
first place? When QEMU is being launched the fork is already created
and QEMU doesn't need to know anything about the parent.

Tamas

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel