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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

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.

Roger.

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